This adds legalization for scalar G_CTTZ and G_CTTZ_ZERO_UNDEF. Vector support requires handling vector G_BITREVERSE, which I haven't gotten around to yet.
For G_CTTZ_ZERO_UNDEF, we just lower it to G_CTTZ.
For G_CTTZ, we match SelectionDAG's lowering to a G_BITREVERSE + G_CTLZ.
(With this patch, we have slightly worse codegen than SDAG for types smaller than s32; it seems like we're missing a combine.)
Also, this adds in a function to build G_BITREVERSE to MachineIRBuilder.