tree-optimization/99954 - redo loop distribution memcpy recognition fix

tree-optimization/99954 - redo loop distribution memcpy recognition fix

The following revisits the fix for PR99954 which was observed as
causing missed memcpy recognition and instead using memmove for
non-aliasing copies.  While the original fix mitigated bogus
recognition of memcpy the root cause was not properly identified.
The root cause is dr_analyze_indices "failing" to handle union
references and leaving the DRs indices in a state that's not correctly
handled by dr_may_alias.  The following mitigates this there
appropriately, restoring memcpy recognition for non-aliasing copies.

This makes us run into a latent issue in ptr_deref_may_alias_decl_p
when the pointer is something like &MEM[0].a in which case we fail
to handle non-SSA name pointers.  Add code similar to what we have
in ptr_derefs_may_alias_p.

	PR tree-optimization/99954
	* tree-data-ref.cc (dr_may_alias_p): For bases that are
	not completely analyzed fall back to TBAA and points-to.
	* tree-loop-distribution.cc
	(loop_distribution::classify_builtin_ldst): When there
	is no dependence again classify as memcpy.
	* tree-ssa-alias.cc (ptr_deref_may_alias_decl_p): Verify
	the pointer is an SSA name.

	* gcc.dg/tree-ssa/ldist-40.c: New testcase.