[X86] Improve lowering of fptoui

Invert the select condition when masking in the sign bit of a fptoui operation. Also, rather than lowering the sign mask to select/xor and expecting the select to get cleaned up later, directly lower to shift/xor.

Patch by Layton Kifer!

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D90658