This patch introduces an experimental intrinsic for counting the
trailing zero elements in a vector. The intrinsic has generic expansion
in SelectionDAGBuilder, and for AArch64 there is a pattern which matches
to brkb & cntp instructions where SVE is enabled.
The intrinsic has a second operand to indicate whether it returns a valid
result if the first argument is all zero, similar to the existing cttz intrinsic.
These changes have been split out from D158291.