tree-optimization/102659 - really avoid undef overflow in if-conversion

tree-optimization/102659 - really avoid undef overflow in if-conversion

This plugs the remaining hole of POINTER_PLUS_EXPR with undefined
overflow.  Unfortunately we have to go through some lengths to
not put invariant conversions into the loop body since that confuses
the vectorizers gather/scatter discovery which relies on identifying
an invariant component of plus and minus expressions.  We can
emit those in the loop preheader but then we have to accept that
being non-empty when looking for the LOOP_VECTORIZED internal
function call in the vectorizer.

2021-10-14  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/102659
	* tree-if-conv.c (if_convertible_gimple_assign_stmt_p): Also
	rewrite pointer typed undefined overflow operations.
	(predicate_statements): Likewise.  Make sure to emit invariant
	conversions in the preheader.
	* tree-vectorizer.c (vect_loop_vectorized_call): Look through
	non-empty preheaders.
	* tree-data-ref.c (dr_analyze_indices): Strip useless
	conversions to the MEM_REF base type.