Fix MSVC "compiler limit: blocks nested too deeply" error. NFCI.

MSVC only accepts if-else chains up to 127 blocks long. I've had to merge a number of intrinsic cases together to get back below this limit, resulting in some duplication of string matches; this shouldn't cause any notable increase in runtime (and even then only for old IR, nothing that clang currently emits).

llvm-svn: 339666