[UpdateTestChecks] Handle IR variables with a '-' in the name

Summary:
I noticed that clang will emit variables such as %indirect-arg-temp when
running update_cc1_test_checks.py and therefore update_cc1_test_checks.py
wasn't adding FileCheck captures for those variables.

Reviewers: MaskRay

Reviewed By: MaskRay

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D44459

llvm-svn: 327564