update_mir_test_checks: Support adding checks for vreg classes

This is a temporary hack to support adding checks for the "registers:"
block of mir functions. This is necessary to convert a number of tests
so that there's less churn when we change the MIR printer to put the
vreg classes on defs instead of in their own block.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316134 91177308-0d34-0410-b5e6-96231b3b80d8