#139273 introduced a CompressPat to convert qe.e.li to c.li.

let isCompressOnly = true in
def : CompressPat<(QC_E_LI GPRNoX0:$rd, simm6:$imm),
                  (C_LI GPRNoX0:$rd, simm6:$imm)>;

There is a crash when assembling qc.e.li a0, undef.

Immediate thought: Is simm6 appropriate in the CompressPat? Shall we allow immediates only?

But that would rule out compression for qc.e.li x10, foo; foo = 8, which you probably want.
(It assembles to 051f 0000 0008 qc.e.li a0, 0x80000 with this PR, which suggests a bug.)

Anyhow, let's continue.

• RISCVAsmParser::emitToStreamer calls RISCVRVC::compress, which calls RISCVValidateMCOperandForCompress to simm6.
• The verifier passes because MCOp.isBareSymbolRef(); returns true.

  case 3: {
    // simm6
    int64_t Imm;
    if (MCOp.evaluateAsConstantImm(Imm))
      return isInt<6>(Imm);
    return MCOp.isBareSymbolRef();
  }

• RISCVMCCodeEmitter::encodeInstruction calls getBinaryCodeForInstr for the compressed c.li, which failed because there wasn't a defined fixup.
  This PR adds a fixup, which could be used to accept c.li a0, foo; foo = 9 (currently rejected by GAS, but with a fixup this could be made working)

• Introducing a fixup for c.li seems like a solid choice.
• You might want to address the qc.e.li x10, foo; foo = 8 bug.
• I recommend that RISC-V avoids adding more hacks to the generic interface, such as MCInst MCOp.isBareSymbolRef(). [RISCV] Align MCOperandPredicates with AsmParser #146184 actually eliminates some, which I like. Thanks!