[LoongArch] Add codegen support for part of conversion operations

These operations include `sext`, `zext` and `trunc`.

Reference:
https://llvm.org/docs/LangRef.html#conversion-operations

Differential Revision: https://reviews.llvm.org/D127204

Author: SixWeining

llvm/lib/Target/LoongArch/LoongArchISelLowering.cpp

@@ -61,6 +61,7 @@ LoongArchTargetLowering::LoongArchTargetLowering(const TargetMachine &TM,
   }
 
   setOperationAction(ISD::SELECT_CC, GRLenVT, Expand);
+  setOperationAction(ISD::SIGN_EXTEND_INREG, MVT::i1, Expand);
 
   // Compute derived properties from the register classes.
   computeRegisterProperties(STI.getRegisterInfo());