[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323790
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