[X86] More aggressive shuffle mask widening in combineExtractWithShuffle

Use demanded extract index to set most of the shuffle mask to undef, making it easier to widen and peek through.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351013 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed