[mips] Fold duplicate big-endian disassembler tests together.

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

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

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

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

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

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

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

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

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

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

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

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

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

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