Add DAG optimisation for FP16_TO_FP

The FP16_TO_FP node only uses the bottom 16 bits of its input, so the
following pattern can be optimised by removing the AND:

  (FP16_TO_FP (AND op, 0xffff)) -> (FP16_TO_FP op)

This is a common pattern for ARM targets when functions have __fp16
arguments, as they are passed as floats (so that they get passed in the
correct registers), but then bitcast and truncated to ignore the top 16
bits.

llvm-svn: 245832