[CodeView] Micro-optimizations to speed up type merging.

Based on a profile, a couple of hot spots were identified in the
main type merging loop.  The code was simplified, a few loops
were re-arranged, and some outlined functions were inlined.  This
speeds up type merging by a decent amount, shaving around 3-4 seconds
off of a 40 second link in my test case.

Differential Revision: https://reviews.llvm.org/D42559

llvm-svn: 323790