Do not try to use i8 and i16 versions of FP_TO_U/SINT soft float library calls

It appears that neither compiler-rt nor the gnu soft-float libraries actually
implement these conversions. Instead of emitting calls to library functions
that don't exist, handle it similarly to the way we handle i8 -> float and
i16 -> float conversions: call the i32 library function, and adjust the type.

Differential Revision: http://reviews.llvm.org/D15151

llvm-svn: 255643