[AMDGPU] Split unaligned 4 DWORD DS operations

Similarly to 3 DWORD operations it is better for performance
to split unlaligned operations as long a these are at least
DWORD alignmened. Performance data:

```
Using platform: AMD Accelerated Parallel Processing
Using device: gfx900:xnack-

ds_write_b128                      aligned by 16:  4.9 sec
ds_write2_b64                      aligned by 16:  5.1 sec
ds_write2_b32 * 2                  aligned by 16:  5.5 sec
ds_write_b128                      aligned by  1:  8.1 sec
ds_write2_b64                      aligned by  1:  8.7 sec
ds_write2_b32 * 2                  aligned by  1: 14.0 sec
ds_write_b128                      aligned by  2:  8.1 sec
ds_write2_b64                      aligned by  2:  8.7 sec
ds_write2_b32 * 2                  aligned by  2: 14.0 sec
ds_write_b128                      aligned by  4:  5.6 sec
ds_write2_b64                      aligned by  4:  8.7 sec
ds_write2_b32 * 2                  aligned by  4:  5.6 sec
ds_write_b128                      aligned by  8:  5.6 sec
ds_write2_b64                      aligned by  8:  5.1 sec
ds_write2_b32 * 2                  aligned by  8:  5.6 sec
ds_read_b128                       aligned by 16:  3.8 sec
ds_read2_b64                       aligned by 16:  3.8 sec
ds_read2_b32 * 2                   aligned by 16:  4.0 sec
ds_read_b128                       aligned by  1:  4.6 sec
ds_read2_b64                       aligned by  1:  8.1 sec
ds_read2_b32 * 2                   aligned by  1: 14.0 sec
ds_read_b128                       aligned by  2:  4.6 sec
ds_read2_b64                       aligned by  2:  8.1 sec
ds_read2_b32 * 2                   aligned by  2: 14.0 sec
ds_read_b128                       aligned by  4:  4.6 sec
ds_read2_b64                       aligned by  4:  8.1 sec
ds_read2_b32 * 2                   aligned by  4:  4.0 sec
ds_read_b128                       aligned by  8:  4.6 sec
ds_read2_b64                       aligned by  8:  3.8 sec
ds_read2_b32 * 2                   aligned by  8:  4.0 sec

Using platform: AMD Accelerated Parallel Processing
Using device: gfx1030

ds_write_b128                      aligned by 16:  6.2 sec
ds_write2_b64                      aligned by 16:  7.1 sec
ds_write2_b32 * 2                  aligned by 16:  7.6 sec
ds_write_b128                      aligned by  1: 24.1 sec
ds_write2_b64                      aligned by  1: 25.2 sec
ds_write2_b32 * 2                  aligned by  1: 43.7 sec
ds_write_b128                      aligned by  2: 24.1 sec
ds_write2_b64                      aligned by  2: 25.1 sec
ds_write2_b32 * 2                  aligned by  2: 43.7 sec
ds_write_b128                      aligned by  4: 14.4 sec
ds_write2_b64                      aligned by  4: 25.1 sec
ds_write2_b32 * 2                  aligned by  4:  7.6 sec
ds_write_b128                      aligned by  8: 14.4 sec
ds_write2_b64                      aligned by  8:  7.1 sec
ds_write2_b32 * 2                  aligned by  8:  7.6 sec
ds_read_b128                       aligned by 16:  6.2 sec
ds_read2_b64                       aligned by 16:  6.3 sec
ds_read2_b32 * 2                   aligned by 16:  7.5 sec
ds_read_b128                       aligned by  1: 12.5 sec
ds_read2_b64                       aligned by  1: 24.0 sec
ds_read2_b32 * 2                   aligned by  1: 43.6 sec
ds_read_b128                       aligned by  2: 12.5 sec
ds_read2_b64                       aligned by  2: 24.0 sec
ds_read2_b32 * 2                   aligned by  2: 43.6 sec
ds_read_b128                       aligned by  4: 12.5 sec
ds_read2_b64                       aligned by  4: 24.0 sec
ds_read2_b32 * 2                   aligned by  4:  7.5 sec
ds_read_b128                       aligned by  8: 12.5 sec
ds_read2_b64                       aligned by  8:  6.3 sec
ds_read2_b32 * 2                   aligned by  8:  7.5 sec
```

Differential Revision: https://reviews.llvm.org/D123634