MIPS/opcodes: Do not use CP0 register names for control registers

MIPS/opcodes: Do not use CP0 register names for control registers

The CP0 control register set has never been defined, however encodings
for the CFC0 and CTC0 instructions remained available for implementers
up until the MIPS32 ISA declared them invalid and causing the Reserved
Instruction exception[1].  Therefore we handle them for both assembly
and disassembly, however in the latter case the names of CP0 registers
from the regular set are incorrectly printed if named registers are
requested.  This is because we do not define separate operand classes
for coprocessor regular and control registers respectively, which means
the disassembler has no way to tell the two cases apart.  Consequently
nonsensical disassembly is produced like:

	cfc0	v0,c0_random

Later the MIPSr5 ISA reused the encodings for XPA ASE MFHC0 and MTHC0
instructions[2] although it failed to document them in the relevant
opcode table until MIPSr6 only.

Correct the issue then by defining a new register class, OP_REG_CONTROL,
and corresponding operand codes, `g' and `y' for the two positions in
the machine instruction a control register operand can take.  Adjust the
test cases affected accordingly.

While at it swap the regular MIPS opcode table "cfc0" and "ctc0" entries
with each other so that they come in the alphabetical order.

References:

[1] "MIPS32 Architecture For Programmers, Volume II: The MIPS32
    Instruction Set", MIPS Technologies, Inc., Document Number: MD00086,
    Revision 1.00, August 29, 2002, Table A-9 "MIPS32 COP0 Encoding of
    rs Field", p. 242

[2] "MIPS Architecture For Programmers, Volume II-A: The MIPS32
    Instruction Set", MIPS Technologies, Inc., Document Number: MD00086,
    Revision 5.04, December 11, 2013, Section 3.2 "Alphabetical List of
    Instructions", pp. 195, 216

	include/
	* opcode/mips.h: Document `g' and `y' operand codes.
	(mips_reg_operand_type): Add OP_REG_CONTROL enumeration
	constant.

	gas/
	* tc-mips.c (convert_reg_type) <OP_REG_CONTROL>: New case.
	(macro) <M_TRUNCWS, M_TRUNCWD>: Use the `g' rather than `G'
	operand code.

	opcodes/
	* mips-dis.c (print_reg) <OP_REG_COPRO>: Move control register
	handling code over to...
	<OP_REG_CONTROL>: ... this new case.
	* mips-opc.c (decode_mips_operand) <'g', 'y'>: New cases.
	(mips_builtin_opcodes): Update "cfc1", "ctc1", "cttc1", "cttc2",
	"cfc0", "ctc0", "cfc2", "ctc2", "cfc3", and "ctc3" entries
	replacing the `G' operand code with `g'.  Update "cftc1" and
	"cftc2" entries replacing the `E' operand code with `y'.
	* micromips-opc.c (decode_micromips_operand) <'g'>: New case.
	(micromips_opcodes): Update "cfc1", "cfc2", "ctc1", and "ctc2"
	entries replacing the `G' operand code with `g'.

	binutils/
	* testsuite/binutils-all/mips/mips-xpa-virt-1.d: Correct CFC0
	operand disassembly.
	* testsuite/binutils-all/mips/mips-xpa-virt-3.d: Likewise.