As suggested by Richard B., this patch makes vect_is_simple_use check
whether a defining statement has been replaced by a pattern statement,
and if so returns the pattern statement instead.

The reason for doing this is that the main patch for PR85694
makes over_widening handle more general cases.  These over-widened
patterns can still be useful when matching later statements;
e.g. an overwidened MULT_EXPR could be the input to a DOT_PROD_EXPR.

The patch doesn't do anything with the STMT_VINFO_IN_PATTERN_P checks
in vect_recog_over_widening_pattern or vect_recog_widen_shift_pattern
since later patches rewrite them anyway.

Doing this fixed an XFAIL in vect-reduc-dot-u16b.c.

2018-06-30  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-loop.c (vectorizable_reduction): Assert that the
	phi is not a pattern statement and has not been replaced by
	a pattern statement.
	* tree-vect-patterns.c (type_conversion_p): Don't check
	STMT_VINFO_IN_PATTERN_P.
	(vect_recog_vector_vector_shift_pattern): Likewise.
	(vect_recog_dot_prod_pattern): Expect vect_is_simple_use to return
	the pattern statement rather than the original statement; check
	directly for a WIDEN_MULT_EXPR here.
	* tree-vect-slp.c (vect_get_and_check_slp_defs): Expect
	vect_is_simple_use to return the pattern statement rather
	than the original statement; use is_pattern_stmt_p to check
	for such a pattern statement.
	* tree-vect-stmts.c (process_use): Expect vect_is_simple_use
	to return the pattern statement rather than the original statement;
	don't do the same transformation here.
	(vect_is_simple_use): If the defining statement has been replaced
	by a pattern statement, return the pattern statement instead.
	Remove the corresponding (local) transformation from the vectype
	overload.

gcc/testsuite/
	* gcc.dg/vect/vect-reduc-dot-u16b.c: Remove xfail and update the
	test for vectorization along the lines described in the comment.

From-SVN: r262273