[SelectionDAG] Add support for FP_ROUND in WidenVectorOperand.

Summary:
This is used on AMDGPU for rounding from v3f64 (which is illegal) to
v3f32 (which is legal).

Subscribers: jvesely, nhaehnle, tpr, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69339