[TLI] SimplifyMultipleUseDemandedBits - remove insert_subvector(undef, x, 0) fold

SimplifyMultipleUseDemandedBits shouldn't be creating general nodes on the fly, it should mainly just peek through them (although we do currently allow creation of new bitcasts and constant folding).

This is mostly a win - by avoiding new nodes we avoid a lot of hasOneUse limitations inside x86 shuffle combining - the main regressions I've noticed are where we've ended up with multiple insert_subvector(undef, x, 0) nodes, widening x to different vector widths - that should hopefully be improved when we remove the last of the vector widening from combineX86ShufflesRecursively for Issue #45319