Improve __builtin_popcount* (x) == 1 generation if x is known != 0 [PR90693]

Improve __builtin_popcount* (x) == 1 generation if x is known != 0 [PR90693]

We expand __builtin_popcount* (x) == 1 as
x ^ (x - 1) > x - 1, either unconditionally in tree-ssa-math-opts.cc
if we don't have a direct optab support for popcount, or during
expansion where we compare the costs of comparison of the popcount
against one vs. the above expression.
As mentioned in the PR, if we know from ranger that the argument is
not zero, we can emit x & (x - 1) == 0 test which is same number of
GIMPLE statements, but on many targets cheaper (e.g. whenever an AND
instruction can also set flags on whether result was zero or not).

The following patch does that.

2024-01-05  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/90693
	* tree-ssa-math-opts.cc (match_single_bit_test): If
	tree_expr_nonzero_p (arg), remember it in the second argument to
	IFN_POPCOUNT or lower it as arg & (arg - 1) == 0 rather than
	arg ^ (arg - 1) > arg - 1.
	* internal-fn.cc (expand_POPCOUNT): If second argument to
	IFN_POPCOUNT suggests arg is non-zero, try to expand it as
	arg & (arg - 1) == 0 rather than arg ^ (arg - 1) > arg - 1.

	* gcc.target/i386/pr90693-2.c: New test.