match.pd transforms (A&C)|(B&~C) to ((A^B)&C)^B, which is fewer
operations if C is not const (and it is not on simple tests at least,
this transform is done very early already).
Various processors have "insert" instructions that can do this, but
combine cannot build those from the xor-and-xor, especially it has no
chance at all to do that if A or B or multiple instructions as well
(on PowerPC, the rl[ws]imi instructions that can do this with a rotate,
or a simple shift with appropriate C; other ISAs have similar insns).
This patch makes RTL simplify transform (xor (and (xor A B) C) B) back
to (ior (and A C) (and B ~C)) for constant C (and similar with A instead
of B for that last term).