[llvm] [MIRParser] Allow undef subreg defs in SSA form (PR #189025)

[Jay Foad via llvm-commits]

jayfoad wrote:

> The undef subreg stuff is somewhat questionable. We are technically defining the full register hence this is SSA-compliant, but we only find out this by looking at the undef flag, which means that every pass needs to know about that. I would rather that we reject subreg def while we're in SSA, but I'm not going to hold this PR.

I'm happy to change X86 GlobalISel and ban all subreg defs in SSA, if that's the consensus.

https://github.com/llvm/llvm-project/pull/189025