[llvm] [PowerPC][SDAG] fix bitcast on ppc_f128 swapping the two halves (PR #208969)

[Folkert de Vries via llvm-commits]

folkertdev wrote:

There is also https://github.com/llvm/llvm-project/blob/main/llvm/test/Transforms/SROA/ppcf128-no-fold.ll which was introduced in https://github.com/llvm/llvm-project/commit/cd5553ed3912f4878ab0bbfffbf6f1d113d1521e.

So that deals with the same issue, but my changes here don't change its assembly so the changes at least don't interfere.

This type turns out to be a bit of a nightmare huh

https://github.com/llvm/llvm-project/pull/208969