AMDGPU: Implement isConstantPhysReg

I don't think any of these registers are used in contexts where this
would do anything yet.

Author: arsenm

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

@@ -1849,3 +1849,16 @@ MCPhysReg SIRegisterInfo::get32BitRegister(MCPhysReg Reg) const {
 
   return AMDGPU::NoRegister;
 }
+
+bool SIRegisterInfo::isConstantPhysReg(MCRegister PhysReg) const {
+  switch (PhysReg) {
+  case AMDGPU::SGPR_NULL:
+  case AMDGPU::SRC_SHARED_BASE:
+  case AMDGPU::SRC_PRIVATE_BASE:
+  case AMDGPU::SRC_SHARED_LIMIT:
+  case AMDGPU::SRC_PRIVATE_LIMIT:
+    return true;
+  default:
+    return false;
+  }
+}

llvm/lib/Target/AMDGPU/SIRegisterInfo.h

@@ -210,6 +210,8 @@ class SIRegisterInfo final : public AMDGPUGenRegisterInfo {
     return isVGPR(MRI, Reg) || isAGPR(MRI, Reg);
   }
 
+  bool isConstantPhysReg(MCRegister PhysReg) const override;
+
   bool isDivergentRegClass(const TargetRegisterClass *RC) const override {
     return !isSGPRClass(RC);
   }