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

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM. I presume the original intention was to use the return values to cache expansions, but I don't think that ever got implemented. 

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