Add MODE_OPAQUE

Add MODE_OPAQUE

After discussion with Richard Sandiford on IRC, he suggested adding a
new mode class MODE_OPAQUE to deal with the problems (PR 96791) we had
been having with POImode/PXImode in powerpc target. This patch is the
accumulation of changes I needed to make to add this and make it useable
for the purposes of what power10 MMA needed.

MODE_OPAQUE modes allow you to have modes for which you can just
define loads and stores. By design, optimization does not expect to
know how to do arithmetic or subregs on these modes. This allows us to
have modes for multi-register vector operations where we don't want to
open Pandora's Box and define general arithmetic operations.

This patch will be followed by a target specific patch to change the
powerpc power10 MMA builtins to use opaque modes, and will also let use
use the vector pair loads/stores defined with that in the inline expansion
of memcpy/memmove, allowing me to fix PR 96791.

gcc/ChangeLog
	PR target/96791
	* mode-classes.def: Add MODE_OPAQUE.
	* machmode.def: Add OPAQUE_MODE.
	* tree.def: Add OPAQUE_TYPE for types that will use
	MODE_OPAQUE.
	* doc/generic.texi: Document OPAQUE_TYPE.
	* doc/rtl.texi: Document MODE_OPAQUE.
	* machmode.h: Add OPAQUE_MODE_P().
	* genmodes.c (complete_mode): Add MODE_OPAQUE.
	(opaque_mode): New function.
	* tree.c (tree_code_size): Add OPAQUE_TYPE.
	* tree.h: Add OPAQUE_TYPE_P().
	* stor-layout.c (int_mode_for_mode): Treat MODE_OPAQUE modes
	like BLKmode.
	* ira.c (find_moveable_pseudos): Treat MODE_OPAQUE modes more
	like integer/float modes here.
	* dbxout.c (dbxout_type): Treat OPAQUE_TYPE like VOID_TYPE.
	* tree-pretty-print.c (dump_generic_node): Treat OPAQUE_TYPE
	like like other types.