[X86][IR] Move the autoupgrading of store intrinsics out of the main nested...

[X86][IR] Move the autoupgrading of store intrinsics out of the main nested if/else chain. This should buy a little more time against the MSVC limit mentioned in PR31034.

The handlers for stores all return at the end of their block so they can be picked off early.

llvm-svn: 289339