[CodeGen] Add support for widening INSERT_SUBVECTOR operands

When attempting to return something like a <vscale x 1 x i32>
type from a function we end up trying to widen the vector by
inserting a <vscale x 1 x i32> subvector into an undefined
<vscale x 4 x i32> vector. However, during legalisation we
then attempt to widen the INSERT_SUBVECTOR operands and hit
an error in WidenVectorOperand.

This patch adds a new WidenVecOp_INSERT_SUBVECTOR function
that currently only supports inserting subvectors into undefined
vectors.

Differential Revision: https://reviews.llvm.org/D102501