[ARM] Fix select_cc lowering for fp16

When lowering a select_cc node where the true and false values are of type f16,
we can't use a general conditional move because the FP16 instructions do not
support conditional execution. Instead, we must ensure that the condition code
is one of the four supported by the VSEL instruction.

Differential revision: https://reviews.llvm.org/D58813

llvm-svn: 355385