[llvm] Remove redundant check-prefix=CHECK from tests. NFC.

Reviewers: t.p.northover, oren_ben_simhon, niravd, mcrosier

Reviewed By: oren_ben_simhon, mcrosier

Subscribers: nhaehnle, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D35466

llvm-svn: 308193