Handle non-grouped stores as single-lane SLP: adjust 'gcc.dg/vect/slp-26.c', GCN

Handle non-grouped stores as single-lane SLP: adjust 'gcc.dg/vect/slp-26.c', GCN

As of commit d34cda720988674bcf8a24267c9e1ec61335d6de
"Handle non-grouped stores as single-lane SLP", we see for
'--target=amdgcn-amdhsa' (tested '-march=gfx908', '-march=gfx1100'):

    PASS: gcc.dg/vect/slp-26.c (test for excess errors)
    PASS: gcc.dg/vect/slp-26.c execution test
    PASS: gcc.dg/vect/slp-26.c scan-tree-dump-times vect "vectorized 1 loops" 1
    [-PASS:-]{+FAIL:+} gcc.dg/vect/slp-26.c scan-tree-dump-times vect "vectorizing stmts using SLP" 1

    gcc.dg/vect/slp-26.c: pattern found 2 times

Apply the same change to 'amdgcn-*-*' as done for 'riscv_v'.

	gcc/testsuite/
	* gcc.dg/vect/slp-26.c: Adjust GCN.