[SelectionDAG][X86] Teach promotion legalization for fp_to_sint/fp_to_uint to...

[SelectionDAG][X86] Teach promotion legalization for fp_to_sint/fp_to_uint to insert an assertsext/assertzext based on the original type

If we put in an assertsext/zext here, we're able to generate better truncate code using pack on pre-avx512 targets.

Similar is already done during type legalization. This is the equivalent for op legalization

Differential Revision: https://reviews.llvm.org/D40591

llvm-svn: 319368