[mips] Sort big-endian disassembler tests by opcode.

llvm/test/MC/Disassembler/Mips/mips1/valid-mips1.txt

llvm/test/MC/Disassembler/Mips/mips2/valid-mips2.txt

llvm/test/MC/Disassembler/Mips/mips3/valid-mips3.txt

llvm/test/MC/Disassembler/Mips/mips32/valid-mips32.txt

llvm/test/MC/Disassembler/Mips/mips32r2/valid-mips32r2.txt

llvm/test/MC/Disassembler/Mips/mips32r3/valid-mips32r3.txt

llvm/test/MC/Disassembler/Mips/mips32r5/valid-mips32r5.txt

llvm/test/MC/Disassembler/Mips/mips32r6/valid-mips32r6.txt

llvm/test/MC/Disassembler/Mips/mips4/valid-mips4.txt

llvm/test/MC/Disassembler/Mips/mips64/valid-mips64.txt

llvm/test/MC/Disassembler/Mips/mips64r2/valid-mips64r2.txt

llvm/test/MC/Disassembler/Mips/mips64r3/valid-mips64r3.txt

llvm/test/MC/Disassembler/Mips/mips64r5/valid-mips64r5.txt

llvm/test/MC/Disassembler/Mips/mips64r6/valid-mips64r6.txt