[ARM] FP16 VSEL codegen

This is a follow up of rL327695 to instruction select more variants of VSELGT
and VSELGE, for which it is necessary to custom lower SELECT.

More work is required in this area, which will be addressed soon:
- more variants need to be regression tested, but this depends on the next point.
- first LowerConstantFP need to be adjusted for fp16 values.

Differential Revision: https://reviews.llvm.org/D45205

llvm-svn: 329788