[x86] try harder to scalarize a vector load with extracted integer op uses

This is a retry of b4b97ec8 - that was reverted because it
could cause miscompiles by illegally reordering memory operations.
A new test based on #53695 is added here to verify we do not have
that same problem.

extract_vec_elt (load X), C --> scalar load (X+C)

As noted in the comment, DAGCombiner has this fold -- and the code in this
patch is adapted from DAGCombiner::scalarizeExtractedVectorLoad() -- but
x86 should benefit even if the loaded vector has other uses as long as we
apply some other x86-specific conditions. The motivating example from #50310
is shown in vec_int_to_fp.ll.

Fixes #50310
Fixes #53695

Differential Revision: https://reviews.llvm.org/D118376