[llvm] [PowerPC] Fix assert exposed by PR 95931 in LowerBITCAST (PR #108062)

[Zaara Syeda via llvm-commits]

syzaara wrote:

> This should probably get backported since #95931 was in #95931 (or I guess that one reverted, since I said in the backport that it seemed low risk but that was apparently erroneous :/)

I see 95931 change is still in release/19.x so it would be good to have this PR back ported as well.

https://github.com/llvm/llvm-project/pull/108062