The experimental_cttz_elts intrinsic currently returns a cost
of 1 for all types, however we know that it currently requires
2 SVE instructions when lowering this - brkb and cntp. Both of
these instructions have a throughput that is half of a basic
vector instruction such as a vector add. This patch bumps the
cost of this intrinsic up to 4 to reflect two instructions of
lower throughput.
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