[VectorCombine] foldBitcastShuffle - peek through any residual bitcasts before...

[VectorCombine] foldBitcastShuffle - peek through any residual bitcasts before creating a new bitcast on top (#86119)

Encountered while working on #67803, wading through the chains of bitcasts that SSE intrinsics introduces - this patch helps prevents cases where the bitcast chains aren't cleared out and we can't perform further combines until after InstCombine/InstSimplify has run.