[RISCV] Implement isLegalICmpImmediate

I'm unable to construct a representative test case that demonstrates the 
advantage, but it seems sensible to report accurate target-specific 
information regardless.

llvm-svn: 330938
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.h b/llvm/lib/Target/RISCV/RISCVISelLowering.h
index 7e4a134..dccd031 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.h
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.h
@@ -42,6 +42,7 @@
   bool isLegalAddressingMode(const DataLayout &DL, const AddrMode &AM, Type *Ty,
                              unsigned AS,
                              Instruction *I = nullptr) const override;
+  bool isLegalICmpImmediate(int64_t Imm) const override;
   bool isLegalAddImmediate(int64_t Imm) const override;
 
   // Provide custom lowering hooks for some operations.