[llvm] [VectorCombine] Preserves the maximal legal FPMathFlags during foldShuffleToIdentity (PR #94295)

[Henry Jiang via llvm-commits]

mustartt wrote:

@davemgreen Good idea on using `propagateIRFlags`, it handles most of the flags now I think.

https://github.com/llvm/llvm-project/pull/94295