[llvm] [CodeGen] Let RDA recompute live-ins. (PR #166773)

[Sam Elliott via llvm-commits]

lenary wrote:

IIRC, RDA doesn't track function live-ins particularly well. `ReachingDefAnalysis::getGlobalReachingDefs(...)` doesn't model function live-ins at all, so you end up needing to check live-in-ness of a register separately.

I've worked around this in the past, but this issue would give me pause for using RDA in PEI, before the issue is resolved.

https://github.com/llvm/llvm-project/pull/166773