[AMDGPU] Add test coverage to ensure first regallocfast only allocates SGPR

[jrbyrnes]

Summary

Register allocation is split into two passes, and the expected behavior is that the first pass only should only work on virtual SGPRs. Whereas the second pass works on virtual VGPRs. This adds a test case which breaks if the first pass allocates VGPRs.

[arsenm]

llvm/test/CodeGen/AMDGPU/fastregalloc-sgpr-only.mir

3

Should specify run with regallocfast,0

[jrbyrnes]

Specify regallocfast,0. Add sgpr allocation test

llvm/test/CodeGen/AMDGPU/fastregalloc-sgpr-only.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -O0 -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -start-before=regallocfast,0 --stop-after=regallocfast,0 -o - %s | FileCheck -check-prefix=GCN %s
+
+---
+name: copy_vgpr_allocation
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $vgpr0, $vgpr1, $vgpr4, $vgpr5
+    ; GCN-LABEL: name: copy_vgpr_allocation
+    ; GCN: liveins: $vgpr0, $vgpr1, $vgpr4, $vgpr5
+    ; GCN-NEXT: {{  $}}
+    ; GCN-NEXT: [[COPY:%[0-9]+]]:vreg_64 = COPY killed $vgpr0_vgpr1
+    ; GCN-NEXT: FLAT_STORE_DWORDX2 killed $vgpr4_vgpr5, [[COPY]], 0, 0, implicit $exec, implicit $flat_scr
+    %14:vreg_64 = COPY $vgpr0_vgpr1
+    FLAT_STORE_DWORDX2 $vgpr4_vgpr5, %14:vreg_64, 0, 0, implicit $exec, implicit $flat_scr
+...
+
+
+---
+name: copy_sgpr_allocation
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $sgpr0, $sgpr1, $sgpr4
+    ; GCN-LABEL: name: copy_sgpr_allocation
+    ; GCN: liveins: $sgpr0, $sgpr1, $sgpr4
+    ; GCN-NEXT: {{  $}}
+    ; GCN-NEXT: S_STORE_DWORD_SGPR killed $sgpr4, killed renamable $sgpr0_sgpr1, undef $m0, 0
+    %14:sreg_64 = COPY $sgpr0_sgpr1
+    S_STORE_DWORD_SGPR $sgpr4, %14:sreg_64, undef $m0, 0
+...