[X86] Add extra-uses on the mask of pattern c of extract-{low,}bits.ll tests

Summary:
Because of the D48768, that pattern is always unfolded into pattern d,
thus we had no test coverage.

Reviewers: RKSimon, craig.topper

Reviewed By: craig.topper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D53574

llvm-svn: 345583