Avoid pointer compares on TYPE_MAIN_VARIANT in TBAA

Avoid pointer compares on TYPE_MAIN_VARIANT in TBAA

while building more testcases for ipa-icf I noticed that there are two places
in aliasing code where we still compare TYPE_MAIN_VARIANT for pointer equality.
This is not good idea for LTO since type merging may not happen for example
when in one unit pointed to type is forward declared while in other it is fully
defined.  We have same_type_for_tbaa for that.

Bootstrapped/regtested x86_64-linux, OK?

gcc/ChangeLog:

	* alias.cc (reference_alias_ptr_type_1): Use view_converted_memref_p.
	* alias.h (view_converted_memref_p): Declare.
	* tree-ssa-alias.cc (view_converted_memref_p): Export.
	(ao_compare::compare_ao_refs): Use same_type_for_tbaa.