update_mir_test_checks: Be careful about replacing entire vregs

Previously, this could end up replacing a vreg like %14 with
[[VREG1]]4, where VREG1 was the match for %1. That's obviously not
correct, though it hasn't actually come up in any tests I've converted
so far.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317509 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed