[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.

0 files changed, 0 insertions, 0 deletions