vect: Fix single def-use cycle for ifn reductions [PR108608]

vect: Fix single def-use cycle for ifn reductions [PR108608]

The patch that added support for fmin/fmax reductions didn't
handle single def-use cycles.  In some ways, this seems like
going out of our way to make things slower, but that's a
discussion for another day.

gcc/
	PR tree-optimization/108608
	* tree-vect-loop.cc (vect_transform_reduction): Handle single
	def-use cycles that involve function calls rather than tree codes.

gcc/testsuite/
	PR tree-optimization/108608
	* gcc.dg/vect/pr108608.c: New test.
	* gcc.target/aarch64/sve/pr108608-1.c: Likewise.