Force check prof branch_weights consistency in SwitchInstProfUpdateWrapper

This patch turns on the prof branch_weights metadata consistency
check in SwitchInstProfUpdateWrapper.

If this patch causes a failure then please before reverting do report
the IR that hits the assertion and try identifying the pass that
introduces the inconsistency. We have to fix all such passes.

See also the upcoming change https://reviews.llvm.org/D61179
in the Verifier.

Reviewers: davidx, nikic, eraman, reames, chandlerc
Reviewed By: davidx
Differential Revision: https://reviews.llvm.org/D64061

llvm-svn: 368129