[RISCV] Convert C_ADDI_NOP to C_NOP in the assembler. (#112314)

Make it a pseudoinstruction so we can convert it to C_NOP. This makes
the printing from the assembler consistent with what we get from
llvm-objdump.

I tried to do this with an InstAlias, but I don't think it can drop
operands.

Author: topperc

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

@@ -3693,6 +3693,9 @@ bool RISCVAsmParser::processInstruction(MCInst &Inst, SMLoc IDLoc,
   switch (Inst.getOpcode()) {
   default:
     break;
+  case RISCV::PseudoC_ADDI_NOP:
+    emitToStreamer(Out, MCInstBuilder(RISCV::C_NOP));
+    return false;
   case RISCV::PseudoLLAImm:
   case RISCV::PseudoLAImm:
   case RISCV::PseudoLI: {

llvm/lib/Target/RISCV/RISCVInstrInfoC.td

@@ -418,15 +418,11 @@ def C_ADDI : RVInst16CI<0b000, 0b01, (outs GPRNoX0:$rd_wb),
   let Inst{6-2} = imm{4-0};
 }
 
-let hasSideEffects = 0, mayLoad = 0, mayStore = 0 in
-def C_ADDI_NOP : RVInst16CI<0b000, 0b01, (outs GPRX0:$rd_wb),
-                            (ins GPRX0:$rd, immzero:$imm),
-                            "c.addi", "$rd, $imm">,
-                 Sched<[WriteIALU, ReadIALU]> {
-  let Constraints = "$rd = $rd_wb";
-  let Inst{6-2} = 0;
-  let isAsmParserOnly = 1;
-}
+// Alternate syntax for c.nop. Converted to C_NOP by the assembler.
+let hasSideEffects = 0, mayLoad = 0, mayStore = 0, isCodeGenOnly = 0,
+    isAsmParserOnly = 1 in
+def PseudoC_ADDI_NOP : Pseudo<(outs GPRX0:$rd), (ins GPRX0:$rs1, immzero:$imm),
+                              [], "c.addi", "$rd, $imm">;
 
 let hasSideEffects = 0, mayLoad = 0, mayStore = 0, isCall = 1,
     DecoderNamespace = "RISCV32Only_", Defs = [X1],

llvm/test/MC/RISCV/rv32c-valid.s

@@ -147,8 +147,7 @@ c.sub a4, a5
 # CHECK-ASM: encoding: [0x01,0x00]
 # CHECK-NO-EXT:  error: instruction requires the following: 'C' (Compressed Instructions) or 'Zca' (part of the C extension, excluding compressed floating point loads/stores){{$}}
 c.nop
-# CHECK-ASM: c.addi zero, 0
-# CHECK-OBJ: c.nop
+# CHECK-ASM-AND-OBJ: c.nop
 # CHECK-ASM: encoding: [0x01,0x00]
 # CHECK-NO-EXT:  error: instruction requires the following: 'C' (Compressed Instructions) or 'Zca' (part of the C extension, excluding compressed floating point loads/stores){{$}}
 c.addi x0, 0