[AArch64][GlobalISel] Legalize scalar G_CTTZ + G_CTTZ_ZERO_UNDEF

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.

e.g. https://godbolt.org/z/nPEseYh1s

(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.

Differential Revision: https://reviews.llvm.org/D104065