middle-end: Support not decomposing specific divisions during vectorization.

middle-end: Support not decomposing specific divisions during vectorization.

In plenty of image and video processing code it's common to modify pixel values
by a widening operation and then scale them back into range by dividing by 255.

e.g.:

   x = y / (2 ^ (bitsize (y)/2)-1

This patch adds a new target hook can_special_div_by_const, similar to
can_vec_perm which can be called to check if a target will handle a particular
division in a special way in the back-end.

The vectorizer will then vectorize the division using the standard tree code
and at expansion time the hook is called again to generate the code for the
division.

Alot of the changes in the patch are to pass down the tree operands in all paths
that can lead to the divmod expansion so that the target hook always has the
type of the expression you're expanding since the types can change the
expansion.

gcc/ChangeLog:

	* expmed.h (expand_divmod): Pass tree operands down in addition to RTX.
	* expmed.cc (expand_divmod): Likewise.
	* explow.cc (round_push, align_dynamic_address): Likewise.
	* expr.cc (force_operand, expand_expr_divmod): Likewise.
	* optabs.cc (expand_doubleword_mod, expand_doubleword_divmod):
	Likewise.
	* target.h: Include tree-core.
	* target.def (can_special_div_by_const): New.
	* targhooks.cc (default_can_special_div_by_const): New.
	* targhooks.h (default_can_special_div_by_const): New.
	* tree-vect-generic.cc (expand_vector_operation): Use it.
	* doc/tm.texi.in: Document it.
	* doc/tm.texi: Regenerate.
	* tree-vect-patterns.cc (vect_recog_divmod_pattern): Check for support.
	* tree-vect-stmts.cc (vectorizable_operation): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/vect-div-bitmask-1.c: New test.
	* gcc.dg/vect/vect-div-bitmask-2.c: New test.
	* gcc.dg/vect/vect-div-bitmask-3.c: New test.
	* gcc.dg/vect/vect-div-bitmask.h: New file.