aarch64: Relax type-checking assert [PR126064]

Comparing `arg_type` and `TREE_TYPE (b)` by pointer address causes an
assertion failure when one type is a `typedef` for the other (eg
`uint32x2_t` and `__Uint32x2_t`. Fix by using a more relaxed comparison.

gcc/ChangeLog:

	PR target/126064
	* config/aarch64/aarch64-neon-builtins-base.cc: Compare
	`arg_type` and `TREE_TYPE (b)` using `types_compatible_p` rather
	than comparing their pointer addresses.

gcc/testsuite/ChangeLog:

	PR target/126064
	* gcc.target/aarch64/pr126064.c: New test.