The assume intrinsic is intentionally marked as may reading/writing
memory, to avoid passes moving them around. When flattening the CFG
for predicated blocks, we have to drop the assume calls, as they
are control-flow dependent.
Note that this patch also moves setting State->CFG.PrevVPBB after
we executed the recipes in a VPBB. This way, we have access to the real
VPBB while executing a block.