tree-vect-patterns: One small vect_recog_ctz_ffs_pattern tweak [PR109011]

I've noticed I've made a typo, ifn in this function this late
is always only IFN_CTZ or IFN_FFS, never IFN_CLZ.

Due to this typo, we weren't using the originally intended
.CTZ (X) = .POPCOUNT ((X - 1) & ~X)
but
.CTZ (X) = PREC - .POPCOUNT (X | -X)
instead when we want to emit __builtin_ctz*/.CTZ using .POPCOUNT.
Both compute the same value, both are defined at 0 with the
same value (PREC), both have same number of GIMPLE statements,
but I think the former ought to be preferred, because lots of targets
have andn as a single operation rather than two, and also putting
a -1 constant into a vector register is often cheaper than vector
with broadcast PREC power of two value.

2023-04-20  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/109011
	* tree-vect-patterns.cc (vect_recog_ctz_ffs_pattern): Use
	.CTZ (X) = .POPCOUNT ((X - 1) & ~X) in preference to
	.CTZ (X) = PREC - .POPCOUNT (X | -X).