match.pd: Optimize ffs of known non-zero arg into ctz + 1 [PR94956]

match.pd: Optimize ffs of known non-zero arg into ctz + 1 [PR94956]

The ffs expanders on several targets (x86, ia64, aarch64 at least)
emit a conditional move or similar code to handle the case when the
argument is 0, which makes the code longer.
If we know from VRP that the argument will not be zero, we can (if the
target has also an ctz expander) just use ctz which is undefined at zero
and thus the expander doesn't need to deal with that.

2020-05-08  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/94956
	* match.pd (FFS): Optimize __builtin_ffs* of non-zero argument into
	__builtin_ctz* + 1 if direct IFN_CTZ is supported.

	* gcc.target/i386/pr94956.c: New test.