[RISCV][CodeGen] add assertion to RISCVTargetStreamer getTargetStreamer()

[michaelmaitland]

Summary

X86 and ARM AsmParsers have this same assertion. This assertion provides better reporting when the RISCVTargetStreamer is null and helps to prevent null pointer access.

[bkramer]

Makes sense, thanks!

[michaelmaitland]

I do not have commit access. Can someone please commit for me? Thanks.

llvm/lib/Target/RISCV/AsmParser/RISCVAsmParser.cpp

class RISCVAsmParser : public MCTargetAsmParser {
   SmallVector<ParserOptionsSet, 4> ParserOptionsStack;
   ParserOptionsSet ParserOptions;
 
   SMLoc getLoc() const { return getParser().getTok().getLoc(); }
   bool isRV64() const { return getSTI().hasFeature(RISCV::Feature64Bit); }
   bool isRV32E() const { return getSTI().hasFeature(RISCV::FeatureRV32E); }
 
   RISCVTargetStreamer &getTargetStreamer() {
+    assert(getParser().getStreamer().getTargetStreamer() &&
+           "do not have a target streamer");
     MCTargetStreamer &TS = *getParser().getStreamer().getTargetStreamer();
     return static_cast<RISCVTargetStreamer &>(TS);
   }
 
   unsigned validateTargetOperandClass(MCParsedAsmOperand &Op,
                                       unsigned Kind) override;
 
   bool generateImmOutOfRangeError(OperandVector &Operands, uint64_t ErrorInfo,