[VectorCombine] allow vector loads with mismatched insert type

This is an enhancement to D81766 to allow loading the minimum target
vector type into an IR vector with a different number of elements.

In one of the motivating tests from PR16739, SLP creates <2 x float>
load ops mixed with <4 x float> insert ops, so we want to handle that
pattern in addition to potential oversized vectors created by the
vectorizers.

For now, we are assuming the insert/extract subvector with undef is
free because there is no exact corresponding TTI modeling for that.

Differential Revision: https://reviews.llvm.org/D86160