PR tree-optimization/97546 Bail out of find_bswap_or_nop on non-INTEGER_CST sizes

PR tree-optimization/97546 Bail out of find_bswap_or_nop on non-INTEGER_CST sizes

This patch fixes the ICE in the PR by bailing out of find_bswap_or_nop
on poly_int sizes.
I don't think it intends to handle them and from my reading of the code
it's the most appropriate place to reject them
here rather than in the callers.

Bootstrapped and tested on aarch64-none-linux-gnu.

gcc/
	PR tree-optimization/97546
	* gimple-ssa-store-merging.c (find_bswap_or_nop): Return NULL if
	type is not INTEGER_CST.

gcc/testsuite/
	PR tree-optimization/97546
	* gcc.target/aarch64/sve/acle/general/pr97546.c: New test.