Revert "[X86] Remove uses of the...

Revert "[X86] Remove uses of the -x86-experimental-vector-widening-legalization flag from test/CodeGen/X86/"

This reverts commit 3f572c7b.

The MSan sanitizer buildbot was broken by rL367901. This commit
(rL368079) depends on the broken commit that need to be reverted, and
thus itself is being reverted.

See https://reviews.llvm.org/rL367901 for more information.

llvm-svn: 368106