[CFG] Keep speculatively working around an MSVC compiler crash.

Replace if() with a switch(). Because random changes in the code seem to
suppress the crash.

Story so far:
r325966 - Crash introduced.
r325969 - Speculative fix had no effect.
r325978 - Tried to bisect the offending function, crash suddenly disappeared.
r326016 - After another random change in the code, bug appeared again.

llvm-svn: 326021