Add support to promote f16 to f32

Summary:
This patch adds legalization support to operate on FP16 as a load/store type
and do operations on it as floats.

Tests for ARM are added to test/CodeGen/ARM/fp16-promote.ll

Reviewers: srhines, t.p.northover

Differential Revision: http://reviews.llvm.org/D8755

llvm-svn: 235215