MFC r342592:

Pull in r342397 from upstream llvm trunk (by Amara Emerson):

Revert "Revert r342183 "[DAGCombine] Fix crash when store merging
created an extract_subvector with invalid index.""

Fixed the assertion failure.

Differential Revision: https://reviews.llvm.org/D51831

This fixes 'Assertion failed: ((VT.getVectorNumElements() +
N2C->getZExtValue() <= N1.getValueType().getVectorNumElements()) &&
"Extract subvector overflow!"), function getNode' when building the
multimedia/aom port (with AVX2 enabled).

Reported by: jbeich
PR: 234480