If the mask is made up of elements that form a mask in the higher type we can convert shuffle(bitcast into the bitcast type, simplifying the instruction sequence. A v4i32 2,3,0,1 for example can be treated as a 1,0 v2i64 shuffle. This helps clean up some of the AArch64 concat load combines, along with helping simplify a number of other tests.
The PowerPC combine for v16i8 splat vector loads needed some fixes to keep it working for v16i8 vectors. This improves the handling of v2i64 shuffles to match too, hopefully improving them in general.
Can this can be dropped now?