[MCP] Skip invalidating constant regs during forward propagation

Before this patch, redundant COPY couldn't be removed
for the following case:
  %reg1 = COPY %const-reg
  ... // No use of %reg1 but there is a def/use of %const-reg
  %reg2 = COPY killed %reg1

where this can be optimized to:
  ... // No use of %reg1 but there is a def/use of %const-reg
  %reg2 = COPY %const-reg

This patch allows for such optimization by not
invalidating constant registers. This is safe
even where constant registers are defined, as
architectures like AArch64 and RISCV replace a
dead definition of a GPR with a zero constant
register for certain instructions.

Upstream PR:
llvm/llvm-project#111129

Signed-off-by: Vladimir Radosavljevic <[email protected]>

Author: vladimirradosavljevic

llvm/lib/CodeGen/MachineCopyPropagation.cpp

@@ -885,6 +885,12 @@ void MachineCopyPropagation::ForwardCopyPropagateBlock(MachineBasicBlock &MBB) {
       assert(!Reg.isVirtual() &&
              "MachineCopyPropagation should be run after register allocation!");
 
+      // EVM local begin
+      // Skip invalidating constant registers.
+      if (MRI->isReserved(Reg) && MRI->isConstantPhysReg(Reg))
+        continue;
+      // EVM local end
+
       if (MO.isDef() && !MO.isEarlyClobber()) {
         Defs.push_back(Reg.asMCReg());
         continue;