Replace a list of RegexFindReplaces in a single pass

Replacing in multiple passes, especially if each different
RegexFindReplaces come from different sources (e.g. users), can lead
to some confusing and likely dangerous behavior. Instead, replace in a
single pass by unioning patterns together and re-exec'ing the patterns
to see which one matched.

Change-Id: If218c9e47b3aca8ff665f1da844279aba3cbcc35