[AMDGPU] Refine 64 bit misaligned LDS ops selection

Here is the performance data:
```
Using platform: AMD Accelerated Parallel Processing
Using device: gfx900:xnack-

ds_write_b64                       aligned by  8:  3.2 sec
ds_write2_b32                      aligned by  8:  3.2 sec
ds_write_b16 * 4                   aligned by  8:  7.0 sec
ds_write_b8 * 8                    aligned by  8: 13.2 sec
ds_write_b64                       aligned by  1:  7.3 sec
ds_write2_b32                      aligned by  1:  7.5 sec
ds_write_b16 * 4                   aligned by  1: 14.0 sec
ds_write_b8 * 8                    aligned by  1: 13.2 sec
ds_write_b64                       aligned by  2:  7.3 sec
ds_write2_b32                      aligned by  2:  7.5 sec
ds_write_b16 * 4                   aligned by  2:  7.1 sec
ds_write_b8 * 8                    aligned by  2: 13.3 sec
ds_write_b64                       aligned by  4:  4.6 sec
ds_write2_b32                      aligned by  4:  3.2 sec
ds_write_b16 * 4                   aligned by  4:  7.1 sec
ds_write_b8 * 8                    aligned by  4: 13.3 sec
ds_read_b64                        aligned by  8:  2.3 sec
ds_read2_b32                       aligned by  8:  2.2 sec
ds_read_u16 * 4                    aligned by  8:  4.8 sec
ds_read_u8 * 8                     aligned by  8:  8.6 sec
ds_read_b64                        aligned by  1:  4.4 sec
ds_read2_b32                       aligned by  1:  7.3 sec
ds_read_u16 * 4                    aligned by  1: 14.0 sec
ds_read_u8 * 8                     aligned by  1:  8.7 sec
ds_read_b64                        aligned by  2:  4.4 sec
ds_read2_b32                       aligned by  2:  7.3 sec
ds_read_u16 * 4                    aligned by  2:  4.8 sec
ds_read_u8 * 8                     aligned by  2:  8.7 sec
ds_read_b64                        aligned by  4:  4.4 sec
ds_read2_b32                       aligned by  4:  2.3 sec
ds_read_u16 * 4                    aligned by  4:  4.8 sec
ds_read_u8 * 8                     aligned by  4:  8.7 sec

Using platform: AMD Accelerated Parallel Processing
Using device: gfx1030

ds_write_b64                       aligned by  8:  4.4 sec
ds_write2_b32                      aligned by  8:  4.3 sec
ds_write_b16 * 4                   aligned by  8:  7.9 sec
ds_write_b8 * 8                    aligned by  8: 13.0 sec
ds_write_b64                       aligned by  1: 23.2 sec
ds_write2_b32                      aligned by  1: 23.1 sec
ds_write_b16 * 4                   aligned by  1: 44.0 sec
ds_write_b8 * 8                    aligned by  1: 13.0 sec
ds_write_b64                       aligned by  2: 23.2 sec
ds_write2_b32                      aligned by  2: 23.1 sec
ds_write_b16 * 4                   aligned by  2:  7.9 sec
ds_write_b8 * 8                    aligned by  2: 13.1 sec
ds_write_b64                       aligned by  4: 13.5 sec
ds_write2_b32                      aligned by  4:  4.3 sec
ds_write_b16 * 4                   aligned by  4:  7.9 sec
ds_write_b8 * 8                    aligned by  4: 13.1 sec
ds_read_b64                        aligned by  8:  3.5 sec
ds_read2_b32                       aligned by  8:  3.4 sec
ds_read_u16 * 4                    aligned by  8:  5.3 sec
ds_read_u8 * 8                     aligned by  8:  8.5 sec
ds_read_b64                        aligned by  1: 13.1 sec
ds_read2_b32                       aligned by  1: 22.7 sec
ds_read_u16 * 4                    aligned by  1: 43.9 sec
ds_read_u8 * 8                     aligned by  1:  7.9 sec
ds_read_b64                        aligned by  2: 13.1 sec
ds_read2_b32                       aligned by  2: 22.7 sec
ds_read_u16 * 4                    aligned by  2:  5.6 sec
ds_read_u8 * 8                     aligned by  2:  7.9 sec
ds_read_b64                        aligned by  4: 13.1 sec
ds_read2_b32                       aligned by  4:  3.4 sec
ds_read_u16 * 4                    aligned by  4:  5.6 sec
ds_read_u8 * 8                     aligned by  4:  7.9 sec
```

GFX10 exposes a different pattern for sub-DWORD load/store performance
than GFX9. On GFX9 it is faster to issue a single unaligned load or
store than a fully split b8 access, where on GFX10 even a full split
is better. However, this is a theoretical only gain because splitting
an access to a sub-dword level will require more registers and packing/
unpacking logic, so ignoring this option it is better to use a single
64 bit instruction on a misaligned data with the exception of 4 byte
aligned data where ds_read2_b32/ds_write2_b32 is better.

Differential Revision: https://reviews.llvm.org/D123956