[SPARC] Implement L and H inline asm argument modifiers (#87259)

This adds support for using the L and H argument modifiers for twinword
operands in inline asm code, such as in:

```
%1 = tail call i64 asm sideeffect "rd %pc, ${0:L} ; srlx ${0:L}, 32, ${0:H}", "={o4}"()
```

This is needed by the Linux kernel.

llvm/docs/LangRef.rst

@@ -5557,6 +5557,8 @@ RISC-V:
 
 Sparc:
 
+- ``L``: Print the low-order register of a two-register operand.
+- ``H``: Print the high-order register of a two-register operand.
 - ``r``: No effect.
 
 SystemZ:

llvm/lib/Target/Sparc/SparcAsmPrinter.cpp

@@ -434,6 +434,50 @@ bool SparcAsmPrinter::PrintAsmOperand(const MachineInstr *MI, unsigned OpNo,
     default:
       // See if this is a generic print operand
       return AsmPrinter::PrintAsmOperand(MI, OpNo, ExtraCode, O);
+    case 'L': // Low order register of a twin word register operand
+    case 'H': // High order register of a twin word register operand
+    {
+      const SparcSubtarget &Subtarget = MF->getSubtarget<SparcSubtarget>();
+      const MachineOperand &MO = MI->getOperand(OpNo);
+      const SparcRegisterInfo *RegisterInfo = Subtarget.getRegisterInfo();
+      Register MOReg = MO.getReg();
+
+      Register HiReg, LoReg;
+      if (!SP::IntPairRegClass.contains(MOReg)) {
+        // If we aren't given a register pair already, find out which pair it
+        // belongs to. Note that here, the specified register operand, which
+        // refers to the high part of the twinword, needs to be an even-numbered
+        // register.
+        MOReg = RegisterInfo->getMatchingSuperReg(MOReg, SP::sub_even,
+                                                  &SP::IntPairRegClass);
+        if (!MOReg) {
+          SMLoc Loc;
+          OutContext.reportError(
+              Loc, "Hi part of pair should point to an even-numbered register");
+          OutContext.reportError(
+              Loc, "(note that in some cases it might be necessary to manually "
+                   "bind the input/output registers instead of relying on "
+                   "automatic allocation)");
+          return true;
+        }
+      }
+
+      HiReg = RegisterInfo->getSubReg(MOReg, SP::sub_even);
+      LoReg = RegisterInfo->getSubReg(MOReg, SP::sub_odd);
+
+      Register Reg;
+      switch (ExtraCode[0]) {
+      case 'L':
+        Reg = LoReg;
+        break;
+      case 'H':
+        Reg = HiReg;
+        break;
+      }
+
+      O << '%' << SparcInstPrinter::getRegisterName(Reg);
+      return false;
+    }
     case 'f':
     case 'r':
      break;

llvm/test/CodeGen/SPARC/inlineasm-bad.ll

@@ -11,3 +11,12 @@ entry:
   tail call void asm sideeffect "faddq $0,$1,$2", "{f38},{f0},{f0}"(fp128 0xL0, fp128 0xL0, fp128 0xL0)
   ret void
 }
+
+; CHECK-label:test_twinword_error
+; CHECK: error: Hi part of pair should point to an even-numbered register
+; CHECK: error: (note that in some cases it might be necessary to manually bind the input/output registers instead of relying on automatic allocation)
+
+define i64 @test_twinword_error(){
+  %1 = tail call i64 asm sideeffect "rd %asr5, ${0:L} \0A\09 srlx ${0:L}, 32, ${0:H}", "={i1}"()
+  ret i64 %1
+}

llvm/test/CodeGen/SPARC/inlineasm.ll

@@ -143,3 +143,12 @@ entry:
   %1 = call double asm sideeffect "faddd $1, $2, $0", "=f,f,e"(i64 0, i64 0)
   ret void
 }
+
+; CHECK-label:test_twinword
+; CHECK: rd  %asr5, %i1
+; CHECK: srlx %i1, 32, %i0
+
+define i64 @test_twinword(){
+  %1 = tail call i64 asm sideeffect "rd %asr5, ${0:L} \0A\09 srlx ${0:L}, 32, ${0:H}", "={i0}"()
+  ret i64 %1
+}