[llvm] [X86][SelectionDAG] Fix the Gather's base and index by modifying the Scale value (PR #134979)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

If you're adding additional tests - please can you create a separate PR for them? That way they can be handled first and stop messing up this PR.

https://github.com/llvm/llvm-project/pull/134979