[SelectionDAG] Fixed f16-from-vector promotion problem

Summary:
In the case of an fp_extend of v1f16 to v1f32 where the v1f16 is the
result of a bitcast from i16, avoid creating an illegal fp16_to_fp where
the input is not a vector and the result is a v1f32.

V2: The fix is now to avoid vector scalarization creating a v1->scalar
bitcast.

Reviewers: srhines, t.p.northover

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

Differential Revision: https://reviews.llvm.org/D41126

llvm-svn: 322120