[RISCV] Silence an unused variable warning in release builds [NFC]

[mgrang]

Summary

Also minor cleanups:

1. Avoided multiple calls to Fixup.getKind()
2. Avoided multiple calls to getFixupKindInfo()
3. Removed a redundant return.

[mgrang]

Removed another redundant return in RISCVRegisterInfo.cpp.

[asb]

Looks good to me, thanks! Please go ahead and commit.

lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp

static uint64_t adjustFixupValue(const MCFixup &Fixup, uint64_t Value,
   }
 }
 
 void RISCVAsmBackend::applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
                                  const MCValue &Target,
                                  MutableArrayRef<char> Data, uint64_t Value,
                                  bool IsResolved) const {
   MCContext &Ctx = Asm.getContext();
-  MCFixupKind Kind = Fixup.getKind();
-  unsigned NumBytes = (getFixupKindInfo(Kind).TargetSize + 7) / 8;
+  MCFixupKindInfo Info = getFixupKindInfo(Fixup.getKind());
   if (!Value)
     return; // Doesn't change encoding.
-  MCFixupKindInfo Info = getFixupKindInfo(Fixup.getKind());
   // Apply any target-specific value adjustments.
   Value = adjustFixupValue(Fixup, Value, Ctx);
 
   // Shift the value into position.
   Value <<= Info.TargetOffset;
 
   unsigned Offset = Fixup.getOffset();
+
+#ifndef NDEBUG
+  unsigned NumBytes = (Info.TargetSize + 7) / 8;
   assert(Offset + NumBytes <= Data.size() && "Invalid fixup offset!");
+#endif
 
   // For each byte of the fragment that the fixup touches, mask in the
   // bits from the fixup value.
   for (unsigned i = 0; i != 4; ++i) {
     Data[Offset + i] |= uint8_t((Value >> (i * 8)) & 0xff);
   }
-  return;
 }
 
 std::unique_ptr<MCObjectWriter>
 RISCVAsmBackend::createObjectWriter(raw_pwrite_stream &OS) const {
   return createRISCVELFObjectWriter(OS, OSABI, Is64Bit);
 }
 
 } // end anonymous namespace
 
 MCAsmBackend *llvm::createRISCVAsmBackend(const Target &T,
                                           const MCRegisterInfo &MRI,
                                           const Triple &TT, StringRef CPU,
                                           const MCTargetOptions &Options) {
   uint8_t OSABI = MCELFObjectTargetWriter::getOSABI(TT.getOS());
   return new RISCVAsmBackend(OSABI, TT.isArch64Bit());
 }