Any vector instructions that take a pointer to a base and might modify
it are now flagged with MayModifyStack. Any that actually do modify the stack
(this can be determined by looking at the input types) will produe two dests:
their original result and a new StkPtr. getStackValue calls into VectorEffects
to extract the new type and/or value. The instructions currently assume that
the stack cell they might be modifying is already synced to memory. This may
change in the future when we don't have to do a full SpillStack before the
helpers.
bsimmers
02e0774721