Handle OPAQUE_TYPE specially in verify_type [PR106833]

Handle OPAQUE_TYPE specially in verify_type [PR106833]

As PR106833 shows, cv-qualified opaque type can cause ICE
during LTO.  It exposes that we missd to handle OPAQUE_TYPE
well in type verification.  As Richi pointed out, also
assuming that target will always define TYPE_MAIN_VARIANT
TYPE_CANONICAL for opaque type, this patch is to check
both are OPAQUE_TYPE_P and their modes are of MODE_OPAQUE
class.  Besides, it also checks the only available size
and alignment information.

	PR middle-end/106833

gcc/ChangeLog:

	* tree.cc (verify_opaque_type): New function.
	(verify_type): Call verify_opaque_type for OPAQUE_TYPE.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr106833.c: New test.