middle-end: Fix ICE in poly-int.h due to SLP.

Adds a check to ensure that the input vector arguments
to a function are not variable length. Previously, only the
output vector of a function was checked.

The ICE in question is within the neon-sve-bridge.c test,
and is related to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111268

gcc/ChangeLog:
	PR tree-optimization/111268
	* tree-vect-slp.cc (vectorizable_slp_permutation_1):
	Add variable-length check for vector input arguments
	to a function.