[update_mir_test_checks] Handle MI flags properly

previously we would generate literal check lines w/ no reg-exps for
vregs as MI flags (nsw, ninf, etc.) won't be recognized as a part of MI.

Fixing that. Includes updating the MIR tests that suffered from the
problem.

Reviewed By: bogner

Differential Revision: https://reviews.llvm.org/D68905

llvm-svn: 374829