[x86] Add the test for folding stack spills into pextrw.

[x86] Add the test for folding stack spills into pextrw.

This is a negative test as pextrw doesn't write to all 32-bits of the
spilled GPR. This fold ended up happening when D32684 was landed and
covers the regression that motivated reverting it in r304762.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304763 91177308-0d34-0410-b5e6-96231b3b80d8