tree-optimization/99253 - fix reduction path check

tree-optimization/99253 - fix reduction path check

This fixes an ordering problem with verifying that no intermediate
computations in a reduction path are used outside of the chain.  The
check was disabled for value-preserving conversions at the tail
but whether a stmt was a conversion or not was only computed after
the first use.  The following fixes this by re-ordering things
accordingly.

2021-02-25  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/99253
	* tree-vect-loop.c (check_reduction_path): First compute
	code, then verify out-of-loop uses.

	* gcc.dg/vect/pr99253.c: New testcase.

(cherry picked from commit 1193d05465acd39b6e3c7095274d8351a1e2cd44)