[CodeGen] Add support for widening the result of EXTRACT_SUBVECTOR

When trying to return a type such as <vscale x 1 x i32> from a
function we crash in DAGTypeLegalizer::WidenVecRes_EXTRACT_SUBVECTOR
when attempting to get the fixed number of elements in the vector.

For the simple case we are dealing with, i.e. extracting
<vscale x 1 x i32> from index 0 of input vector <vscale x 4 x i32>
we can simply rely upon existing code that just returns the input.

Differential Revision: https://reviews.llvm.org/D102605