Fix my previous checkin to updatePressureDiffs.

There was one case that we could hit a DebugValue where I didn't think
to check. DebugValues are evil. No checkinable test case, sorry. It's
an obvious fix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189717 91177308-0d34-0410-b5e6-96231b3b80d8