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

[Trevor Gross via llvm-commits]

tgross35 wrote:

This should probably get backported since https://github.com/llvm/llvm-project/pull/95931 was in https://github.com/llvm/llvm-project/pull/95931

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