[AMDGPU] Add instruction selection for i1 to f16 conversion

Summary:
This is required for GPUs with 16 bit instructions where f16 is a
legal register type and hence int_to_fp i1 to f16 is not lowered
by legalizing.

Reviewers: arsenm, nhaehnle

Reviewed By: nhaehnle

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D52018

Change-Id: Ie4c0fd6ced7cf10ad612023c6879724d9ded5851
llvm-svn: 342558