[X86] Extend inputs with elements smaller than i32 to sint_to_fp/uint_to_fp...

[X86] Extend inputs with elements smaller than i32 to sint_to_fp/uint_to_fp before type legalization.

This prevents extends of masks being introduced during lowering where it become difficult to combine them out.

There are a few oddities in here.

We sometimes concatenate two k-registers produced by two compares, sign_extend the combined pair, then extract two halves. This worked better previously because the sign_extend wasn't created until after the fp_to_sint was split which led to a split sign_extend being created.

We probably also need to custom type legalize (v2i32 (sext v2i1)) via widening.

llvm-svn: 324820