phiopt: Optimize x ? __builtin_clz (x) : 32 in GIMPLE [PR97503]

phiopt: Optimize x ? __builtin_clz (x) : 32 in GIMPLE [PR97503]

While we have at the RTL level noce_try_ifelse_collapse combined with
simplify_cond_clz_ctz, that optimization doesn't always trigger because
e.g. on powerpc there is an define_insn to compare a reg against zero and
copy that register to another one and so we end up with a different pseudo
in the simplify_cond_clz_ctz test and punt.

For targets that define C?Z_DEFINED_VALUE_AT_ZERO to 2 for certain modes,
we can optimize it already in phiopt though, just need to ensure that
we transform the __builtin_c?z* calls into .C?Z ifns because my recent
VRP changes codified that the builtin calls are always undefined at zero,
while ifns honor C?Z_DEFINED_VALUE_AT_ZERO equal to 2.
And, in phiopt we already have popcount handling that does pretty much the
same thing, except for always using a zero value rather than the one set
by C?Z_DEFINED_VALUE_AT_ZERO.

So, this patch extends that function to handle not just popcount, but also
clz and ctz.

2020-10-21  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/97503
	* tree-ssa-phiopt.c: Include internal-fn.h.
	(cond_removal_in_popcount_pattern): Rename to ...
	(cond_removal_in_popcount_clz_ctz_pattern): ... this.  Handle not just
	popcount, but also clz and ctz if it has C?Z_DEFINED_VALUE_AT_ZERO 2.

	* gcc.dg/tree-ssa/pr97503.c: New test.