[X86][AutoUpgrade] Make some tweaks to reduce the number of nested if/else in...

[X86][AutoUpgrade] Make some tweaks to reduce the number of nested if/else in the intrinsic upgrade code to avoid an MSVC compiler limit.

MSVC has a nesting limit of around 110-130. An if/else if/else if counts against this next level. The autoupgrade code consists a long chain of these checking matches against strings.

This commit moves some code to a helper function to move out a large if/else chain that was inside of one of the blocks into a separate function. There are more of these we could move or we could change some to lookup tables.

I've also merged together a few similar blocks in the outer chain. This should buy us some margin for a little bit.

llvm-svn: 350564