[SystemZ] Consider VST/VL as SimpleBDXStore/Load (#135623)
Previously `vst` and `vl` were not considered "simple" BDX stores and
loads, leading to, among other things, some opportunities for `mvc`
optimization to be missed.
This PR addresses this and updates some tests to account for additional
`mvc` instructions being emitted.
This is observed to have a neutral or slightly beneficial effect
performance-wise.
