test: Fix FAIL of SAD tests for RVV

RVV didn't explicitly enable SAD optab but we can vectorize it
since loop vectorizer is able to recognize SAD pattern for RVV during analysis.

Current scan check of explicit SAD pattern looks odd,
it should be more reasonable to check recognition of SAD pattern during Loop vectorize analysis.

Other SAD tests like slp-reduc-sad-2.c are checking pattern recognition instead of explicit pattern enable.
Fix SAD dump check to fix the FAILS for RVV.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/slp-reduc-sad.c: Fix check.
	* gcc.dg/vect/vect-reduc-sad.c: Ditto.