Fix fptosi, fptoui from f16 vectors to i8, i16 vectors

Summary:
Convert f16 vectors to corresponding f32 vectors before doing the
conversion to int.

Add tests for v4f16, v8f16.

Reviewers: ab, jmolloy

Subscribers: llvm-commits, srhines

Differential Revision: http://reviews.llvm.org/D14936

llvm-svn: 255263