[clang] [llvm] [PowerPC] Fix codegen for transparent_union function params (PR #101738)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

transparent_union generally means that the value is passed using the convention of the first member of the union.  Why do you need to check for whether the value was originally a transparent union?  Shouldn't the resulting convention be the same either way?

https://github.com/llvm/llvm-project/pull/101738