adjust testcase for now happening epilogue vectorization
gcc.dg/vect/slp-perm-9.c is reported to FAIL with -march=cascadelake
now which is because we now vectorize the epilogue with V2HImode
vectors after the recent change to not scrap too large vector
epilogues during transform but during analysis time.
The following adjusts the testcase to always use the existing alternate
N which avoids epilogue vectorization.
* gcc.dg/vect/slp-perm-9.c: Always use alternate N.