MIPS: Fix encoding for MIPSr6 sigrie instruction.

MIPS: Fix encoding for MIPSr6 sigrie instruction.

The instruction encoding for the MIPS r6 sigrie instruction seems to be
incorrect.  It's currently 0x4170xxxx (which overlaps with ei, di, evp,
and dvp), but should be 0x0417xxxx.  See ISA reference[1][2].

References:

[1] "MIPS Architecture for Programmers Volume II-A: The MIPS32
    Instruction Set Manual", Imagination Technologies, Inc., Document
    Number: MD00086, Revision 6.06, December 15, 2016, Table A.4 "MIPS32
    REGIMM Encoding of rt Field", p. 452

[2] "MIPS Architecture For Programmers Volume II-A: The MIPS64
    Instruction Set Reference Manual", Imagination Technologies, Inc.,
    Document Number: MD00087, Revision 6.06, December 15, 2016, Table
    A.4 "MIPS64 REGIMM Encoding of rt Field", p. 581

	opcodes/
	* mips-opc.c (mips_builtin_opcodes): Correct "sigrie" encoding.

	gas/
	* testsuite/gas/mips/r6.d: Update for "sigrie" encoding fix.
	* testsuite/gas/mips/r6-n32.d: Likewise.
	* testsuite/gas/mips/r6-n64.d: Likewise.