[X86] Make (f80 (sint_to_fp (i16))) use fistps/fisttps instead of...

[X86] Make (f80 (sint_to_fp (i16))) use fistps/fisttps instead of fistpl/fisttpl when SSE is enabled.

When SSE is enabled sint_to_fp with i16 is blindly promoted to i32, but that changes the behavior of f80 conversion.

Move the promotion to i16 to LowerFP_TO_INT so we can limit it based on the floating point type.

llvm-svn: 354003