AMDGPU/SI: Improve MachineSchedModel definition

This patch contains a few improvements to the model, including:

- Using a single resource with a defined buffers size for each memory unit.
- Setting the IssueWidth correctly.
- Fixing latency values for memory instructions.

shader-db stats:

16429 shaders in 3231 tests
Totals:
SGPRS: 318232 -> 312328 (-1.86 %)
VGPRS: 208996 -> 209346 (0.17 %)
Code Size: 7147044 -> 7166440 (0.27 %) bytes
LDS: 83 -> 83 (0.00 %) blocks
Scratch: 1862656 -> 1459200 (-21.66 %) bytes per wave
Max Waves: 49182 -> 49243 (0.12 %)
Wait states: 0 -> 0 (0.00 %)A

Differential Revision: http://reviews.llvm.org/D18453

llvm-svn: 264877