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

[Rohit Aggarwal via llvm-commits]

rohitaggarwal007 wrote:

> Can this be done in `combineGatherScatter` in X86ISelLowering.cpp before type legalization?

Yes, it is possible there also. 
I thought it will be good to fix it while forming the initial DAG and let the combiner do the further optimization. So I followed this approach.

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