[MCP] Skip invalidating def constant regs during forward propagation (#111129)

Before this patch, redundant COPY couldn't be removed for the following
case:
```
  %reg1 = COPY %const-reg
  ... // There is a def of %const-reg
  %reg2 = COPY killed %reg1
```
where this can be optimized to:
```
  ... // There is a def of %const-reg
  %reg2 = COPY %const-reg
```

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

1 files changed, 5 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/MachineCopyPropagation.cpp b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
index 8bcc437..fb4da2c 100644
--- a/llvm/lib/CodeGen/MachineCopyPropagation.cpp
+++ b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
@@ -886,8 +886,11 @@ void MachineCopyPropagation::ForwardCopyPropagateBlock(MachineBasicBlock &MBB) {
              "MachineCopyPropagation should be run after register allocation!");
 
       if (MO.isDef() && !MO.isEarlyClobber()) {
-        Defs.push_back(Reg.asMCReg());
-        continue;
+        // Skip invalidating constant registers.
+        if (!MRI->isConstantPhysReg(Reg)) {
+          Defs.push_back(Reg.asMCReg());
+          continue;
+        }
       } else if (MO.readsReg())
         ReadRegister(Reg.asMCReg(), MI, MO.isDebug() ? DebugUse : RegularUse);
     }