Machine_Mode: Extend machine_mode from 8 to 16 bits

Machine_Mode: Extend machine_mode from 8 to 16 bits

We are running out of the machine_mode(8 bits) in RISC-V backend. Thus
we would like to extend the machine_mode bit size from 8 to 16 bits.
However, it is sensitive to extend the memory size in common structure
like tree or rtx. This patch would like to extend the machine_mode bits
to 16 bits by shrinking, like:

* Swap the bit size of code and machine code in rtx_def.
* Adjust the machine_mode location and spare in tree.

The memory impact of this patch for correlated structure looks like below:

+-------------------+----------+---------+------+
| struct/bytes      | upstream | patched | diff |
+-------------------+----------+---------+------+
| rtx_obj_reference |        8 |      12 |   +4 |
| ext_modified      |        2 |       4 |   +2 |
| ira_allocno       |      192 |     184 |   -8 |
| qty_table_elem    |       40 |      40 |    0 |
| reg_stat_type     |       64 |      64 |    0 |
| rtx_def           |       40 |      40 |    0 |
| table_elt         |       80 |      80 |    0 |
| tree_decl_common  |      112 |     112 |    0 |
| tree_type_common  |      128 |     128 |    0 |
| access_info       |        8 |       8 |    0 |
+-------------------+----------+---------+------+

The tree and rtx related struct has no memory changes after this patch,
and the machine_mode changes to 16 bits already.

Signed-off-by: Pan Li <pan2.li@intel.com>
Co-authored-by: Ju-Zhe Zhong <juzhe.zhong@rivai.ai>
Co-authored-by: Kito Cheng <kito.cheng@sifive.com>
Co-Authored-By: Richard Biener <rguenther@suse.de>
Co-Authored-By: Richard Sandiford <richard.sandiford@arm.com>
gcc/ChangeLog:

	* combine.cc (struct reg_stat_type): Extend machine_mode to 16 bits.
	* cse.cc (struct qty_table_elem): Extend machine_mode to 16 bits
	(struct table_elt): Extend machine_mode to 16 bits.
	(struct set): Ditto.
	* genmodes.cc (emit_mode_wider): Extend type from char to short.
	(emit_mode_complex): Ditto.
	(emit_mode_inner): Ditto.
	(emit_class_narrowest_mode): Ditto.
	* genopinit.cc (main): Extend the machine_mode limit.
	* ira-int.h (struct ira_allocno): Extend machine_mode to 16 bits and
	re-ordered the struct fields for padding.
	* machmode.h (MACHINE_MODE_BITSIZE): New macro.
	(GET_MODE_2XWIDER_MODE): Extend type from char to short.
	(get_mode_alignment): Extend type from char to short.
	* ree.cc (struct ext_modified): Extend machine_mode to 16 bits and
	removed the ATTRIBUTE_PACKED.
	* rtl-ssa/accesses.h: Extend machine_mode to 16 bits, narrow
	* rtl-ssa/internals.inl (rtl_ssa::access_info): Adjust the assignment.
	m_kind to 2 bits and remove m_spare.
	* rtl.h (RTX_CODE_BITSIZE): New macro.
	(struct rtx_def): Swap both the bit size and location between the
	rtx_code and the machine_mode.
	(subreg_shape::unique_id): Extend the machine_mode limit.
	* rtlanal.h: Extend machine_mode to 16 bits.
	* tree-core.h (struct tree_type_common): Extend machine_mode to 16
	bits and re-ordered the struct fields for padding.
	(struct tree_decl_common): Extend machine_mode to 16 bits.