SrcRegOff >= 0 is not needed at all for unsigned.
This fixes the warning:
```
llvm/lib/Target/Mips/MipsSEInstrInfo.cpp: In member function ‘virtual void llvm::MipsSEInstrInfo::copyPhysReg(llvm::MachineBasicBlock&, llvm::MachineBasicBlock::iterator, const llvm::DebugLoc&, llvm::Register, llvm::Register, bool, bool, bool) const’:
llvm/lib/Target/Mips/MipsSEInstrInfo.cpp:245:48: warning: comparison of unsigned expression in ‘>= 0’ is always true [-Wtype-limits]
245 | if (SrcRegOff == DestRegOff && SrcRegOff >= 0 && SrcRegOff <= 31)
| ~~~~~~~~~~^~~~
llvm/lib/Target/Mips/MipsSEInstrInfo.cpp:256:48: warning: comparison of unsigned expression in ‘>= 0’ is always true [-Wtype-limits]
256 | if (SrcRegOff == DestRegOff && SrcRegOff >= 0 && SrcRegOff <= 31)
```
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