[TargetLowering] expandFP_TO_UINT - improve fp16 support

As discussed on D53794, for float types with ranges smaller than the destination integer type, then we should be able to just use a regular FP_TO_SINT opcode.

I thought we'd need to provide MSA test cases for very small integer types as well (fp16 -> i8 etc.), but it turns out that promotion will kick in so they're unnecessary.

Differential Revision: https://reviews.llvm.org/D54703

llvm-svn: 347251