recog: Add a validate_change_xveclen function

recog: Add a validate_change_xveclen function

A later patch wants to be able to use the validate_change machinery
to reduce the XVECLEN of a PARALLEL.  This should be more efficient
than allocating a separate PARALLEL at a possibly distant memory
location, especially since the new PARALLEL would be garbage rtl if
the new pattern turns out not to match.  Combine already pulls this
trick with SUBST_INT.

This patch adds a general helper for doing that.

gcc/
	* recog.h (validate_change_xveclen): Declare.
	* recog.c (change_t::old_len): New field.
	(validate_change_1): Add a new_len parameter.  Conditionally
	replace the XVECLEN of an rtx, avoiding single-element PARALLELs.
	(validate_change_xveclen): New function.
	(cancel_changes): Undo changes made by validate_change_xveclen.