Amend match.pd syntax with force-simplified results

Amend match.pd syntax with force-simplified results

This adds a ! marker to result expressions that should simplify
(and if not fail the simplification).  This can for example be
used like

(simplify
  (plus (vec_cond:s @0 @1 @2) @3)
  (vec_cond @0 (plus! @1 @3) (plus! @2 @3)))

to make the simplification only apply in case both plus operations
in the result end up simplified to a simple operand.

2020-07-31  Richard Biener  <rguenther@suse.de>

	* genmatch.c (expr::force_leaf): Add and initialize.
	(expr::gen_transform): Honor force_leaf by passing
	NULL as sequence argument to maybe_push_res_to_seq.
	(parser::parse_expr): Allow ! marker on result expression
	operations.
	* doc/match-and-simplify.texi: Amend.