vect: Check gather/scatter offset types [PR108316]

The gather/scatter support can over-widen an offset if the target
requires it, but this relies on using a pattern sequence to add
the widening conversion.  That failed in the testcase because an
earlier pattern (bool) took priority.

I think we should allow patterns to be applied to other patterns,
but that's quite an invasive change and isn't suitable for stage 4.
This patch instead punts if the offset type doesn't match the
expected one.

If we switched to using the SLP representation for everything,
we would probably handle both patterns by rewriting the graph,
which should be much easier.

gcc/
	PR tree-optimization/108316
	* tree-vect-stmts.cc (get_load_store_type): When using
	internal functions for gather/scatter, make sure that the type
	of the offset argument is consistent with the offset vector type.

gcc/testsuite/
	PR tree-optimization/108316
	* gcc.dg/vect/pr108316.c: New test.