[NFC][x86][AArch64] Add BEXTR-like test patterns.

Summary: Also, adjust the check prefixes so that we actually get to check the BMI1-only-case.

Reviewers: craig.topper, RKSimon, spatel, javed.absar

Reviewed By: RKSimon

Subscribers: kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D48490

llvm-svn: 342623