Fix build break when building RISCVInstrInfo.cpp with MSVC (llvm#110342)

After llvm#109774 MSVC is failing to build LLVM with the error:

```
llvm\lib\Target\RISCV\RISCVInstrInfo.cpp(782): warning C4018: '<': signed/unsigned mismatch
```

Fix is ensure that the RHS is an unsigned integer.

Author: dpaoliello