Do not ASSERTZEXT for i16 result of bitcast from f16 operand

Summary:
During legalization if i16, do not ASSERTZEXT the result of FP_TO_FP16.
Directly return an FP_TO_FP16 node with return type as the
promote-to-type of i16.

This patch also removes extraneous length check.  This legalization
should be valid even if integer and float types are of different
lengths.

This patch breaks a hard-float test for fp16 args.  The test is changed
to allow a vmov to zero-out the top bits, and also ensure that the
return value is in an FP register.

Reviewers: ab, jmolloy

Subscribers: srhines, llvm-commits

Differential Revision: http://reviews.llvm.org/D15438

llvm-svn: 257184