[GlobalISel][AArch64] Add support for widening G_FCEIL

This adds support for widening G_FCEIL in LegalizerHelper and
AArch64LegalizerInfo. More specifically, it teaches the AArch64 legalizer to
widen G_FCEIL from a 16-bit float to a 32-bit float when the subtarget doesn't
support full FP 16.

This also updates AArch64/f16-instructions.ll to show that we perform the
correct transformation.

llvm-svn: 349927