match.pd: Fix up FFS -> CTZ + 1 optimization [PR97325]

match.pd: Fix up FFS -> CTZ + 1 optimization [PR97325]

And no testcase was included, I'm including one below.

Anyway, this PR and the other CTZ related discussions led me to discover a
bug I've made earlier, CLZ/CTZ builtins have unsigned arguments and e.g.
both the vr-values.cc and now gimple-range.cc code heavily relies on that,
but __builtin_ffs has a signed operand and this optimization was incorrectly
making the operand signed too, so I guess it would greatly confuse VRP in
some cases.

2020-10-09  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/97325
	* match.pd (FFS(nonzero) -> CTZ(nonzero) + 1): Cast argument to
	corresponding unsigned type.

	* gcc.c-torture/execute/pr97325.c: New test.