[llvm] Update the base and index value for masked gather (PR #130920)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

I'd much prefer to see this fixed in x86 in combineGatherScatter, where there's already a lot of similar code - the TLI callback seems too x86 specific tbh.

https://github.com/llvm/llvm-project/pull/130920