[llvm] [ExpandVectorPredication] Change function returns from Value* to bool. NFC (PR #157199)

[Craig Topper via llvm-commits]

topperc wrote:

> LGTM. I presume the original intention was to use the return values to cache expansions, but I don't think that ever got implemented. We can probably also rename CachingVPExpander -> VPExpander

That was my guess too but the replaceOperation call feels like almost a better place to do the caching anyway.

https://github.com/llvm/llvm-project/pull/157199