MIPS16: Remove "extended" BREAK/SDBBP handling
    
    Remove special casing for the `6' operand code used for the embedded
    trap code of the BREAK and the SDBBP instructions to support supposedly
    extended forms of these instructions.
    
    According to all versions of the MIPS16 ASE specifications these
    instructions are not extensible [1][2][3][4][5][7][8][10][11], and as
    from revision 2.50 of the MIPS16e ASE specifications it has been further
    clarified what was previously implied, that non-extesiable instructions
    when preceded with an EXTEND prefix must cause a Reserved Instruction
    exception [5][6][9][10].
    
    Therefore supposedly extended BREAK and SDBBP instructions do not serve
    their purpose anymore as they do not cause a Bp and a Debug exception
    respectively and supporting these forms in disassembly only causes
    confusion.
    
    References:
    
    [1] "Product Description, MIPS16 Application-Specific Extension",
        Version 1.3, MIPS Technologies, Inc., 970130, Table 3. "MIPS16
        Instruction Set Summary", p. 5
    
    [2] same, Table 5 "RR Minor Opcodes (RR-type instructions)", p.10
    
    [3] same, Table 18. "Extendable MIPS16 Instructions", p. 24
    
    [4] "MIPS32 Architecture for Programmers, Volume IV-a: The MIPS16e
        Application-Specific Extension to the MIPS32 Architecture", MIPS
        Technologies, Inc., Document Number: MD00076, Revision 2.63, July
        16, 2013, Table 3.8 "MIPS16e Special Instructions", p. 38
    
    [5] same, Section 3.11 "MIPS16e Extensible Instructions, p. 41
    
    [6] same, Table 3.15 "MIPS16e Extensible Instructions", p. 41
    
    [7] same, Table 3.24 "MIPS16e RR Encoding of the Funct Field", p. 49
    
    [8] "MIPS64 Architecture for Programmers, Volume IV-a: The MIPS16e
        Application-Specific Extension to the MIPS64 Architecture", MIPS
        Technologies, Inc., Document Number: MD00077, Revision 2.60, June
        25, 2008, Table 1.8 "MIPS16e Special Instructions", p. 39
    
    [9] same, Section 1.11 "MIPS16e Extensible Instructions", p. 42
    
    [10] same, Table 1.15 "MIPS16e Extensible Instructions", pp. 42-43
    
    [11] same, Table 1.24 "MIPS16e RR Encoding of the Funct Field", p. 50
    
    	gas/
    	* config/tc-mips.c (match_mips16_insn): Remove the `6' operand
    	code special case and its associated comment.
    
    	opcodes/
    	* mips16-opc.c (decode_mips16_operand) <'6'>: Remove extended
    	encoding support.
    
    (cherry picked from commit b2805ed55456cea2694d31fc8627cca17120267b)