AMDGPU/SI: SGPR accounting in getSIProgramInfo must ignore exec_lo/hi

[nhaehnle]

Summary

A shader stored the live mask (initial exec mask) in an SGPR which was then
spilled during register allocation. The allocator quite reasonably
optimized turned the spill into

v_writelane_b32 %vgpr, exec_lo, N
v_writelane_b32 %vgpr, exec_hi, N+1

at the beginning of the shader, confusing the SGPR accounting.

No test case, because si-sgpr-spill.ll together with an upcoming patch for
WQM handling exhibits the problem.

[arsenm]

LGTM. Although we should be able to remove finding the number of used registers this way, and can use the register info queries like is done in FrameLowering already

lib/Target/AMDGPU/AMDGPUAsmPrinter.cpp

for (const MachineInstr &MI : MBB) {
         bool isSGPR = false;
 
         if (!MO.isReg())
           continue;
 
         unsigned reg = MO.getReg();
         switch (reg) {
         case AMDGPU::EXEC:
+        case AMDGPU::EXEC_LO:
+        case AMDGPU::EXEC_HI:
         case AMDGPU::SCC:
         case AMDGPU::M0:
           continue;
 
         case AMDGPU::VCC:
         case AMDGPU::VCC_LO:
         case AMDGPU::VCC_HI:
           VCCUsed = true;