test: Adapt slp-26.c check for RVV

test: Adapt slp-26.c check for RVV

Fix FAILs:
FAIL: gcc.dg/vect/slp-26.c -flto -ffat-lto-objects  scan-tree-dump-times vect "vectorized 0 loops" 1
FAIL: gcc.dg/vect/slp-26.c -flto -ffat-lto-objects  scan-tree-dump-times vect "vectorizing stmts using SLP" 0
FAIL: gcc.dg/vect/slp-26.c scan-tree-dump-times vect "vectorized 0 loops" 1
FAIL: gcc.dg/vect/slp-26.c scan-tree-dump-times vect "vectorizing stmts using SLP" 0

Since RVV is able to vectorize it with VLS modes like amdgcn.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/slp-26.c: Adapt for RVV.