[LoongArch][NFC] Fix the operand constraint of AMCAS instructions

The `rd` operand of AMCAS instructions is both read and written, because
of the nature of compare-and-swap operations, but currently it is not
declared as such. Fix it for upcoming codegen enablement changes. No
functional change.

While at it, restore vertical alignment for the definition lines.

Suggested-by: tangaac <[email protected]>
Link: #114398 (comment)

Author: xen0n

llvm/lib/Target/LoongArch/AsmParser/LoongArchAsmParser.cpp

@@ -1562,7 +1562,8 @@ unsigned LoongArchAsmParser::checkTargetMatchPredicate(MCInst &Inst) {
   unsigned Opc = Inst.getOpcode();
   switch (Opc) {
   default:
-    if (Opc >= LoongArch::AMADD_D && Opc <= LoongArch::AMXOR_W) {
+    if (Opc >= LoongArch::AMADD_D && Opc <= LoongArch::AMXOR_W &&
+        !(Opc >= LoongArch::AMCAS_B && Opc <= LoongArch::AMCAS__DB_W)) {
       MCRegister Rd = Inst.getOperand(0).getReg();
       MCRegister Rk = Inst.getOperand(1).getReg();
       MCRegister Rj = Inst.getOperand(2).getReg();

llvm/lib/Target/LoongArch/LoongArchInstrInfo.td

@@ -715,6 +715,11 @@ class AM_3R<bits<32> op>
     : Fmt3R<op, (outs GPR:$rd), (ins GPR:$rk, GPRMemAtomic:$rj),
             "$rd, $rk, $rj">;
 
+let hasSideEffects = 0, mayLoad = 1, mayStore = 1, Constraints = "$rd = $rd_wb" in
+class AMCAS_3R<bits<32> op>
+    : Fmt3R<op, (outs GPR:$rd_wb), (ins GPR:$rd, GPR:$rk, GPRMemAtomic:$rj),
+            "$rd, $rk, $rj">;
+
 let hasSideEffects = 0, mayLoad = 1, mayStore = 0 in {
 class LLBase<bits<32> op>
     : Fmt2RI14<op, (outs GPR:$rd), (ins GPR:$rj, simm14_lsl2:$imm14),
@@ -1024,14 +1029,14 @@ def AMMAX__DB_WU : AM_3R<0x38700000>;
 def AMMAX__DB_DU : AM_3R<0x38708000>;
 def AMMIN__DB_WU : AM_3R<0x38710000>;
 def AMMIN__DB_DU : AM_3R<0x38718000>;
-def AMCAS_B     : AM_3R<0x38580000>;
-def AMCAS_H     : AM_3R<0x38588000>;
-def AMCAS_W     : AM_3R<0x38590000>;
-def AMCAS_D     : AM_3R<0x38598000>;
-def AMCAS__DB_B     : AM_3R<0x385a0000>;
-def AMCAS__DB_H     : AM_3R<0x385a8000>;
-def AMCAS__DB_W     : AM_3R<0x385b0000>;
-def AMCAS__DB_D     : AM_3R<0x385b8000>;
+def AMCAS_B      : AMCAS_3R<0x38580000>;
+def AMCAS_H      : AMCAS_3R<0x38588000>;
+def AMCAS_W      : AMCAS_3R<0x38590000>;
+def AMCAS_D      : AMCAS_3R<0x38598000>;
+def AMCAS__DB_B  : AMCAS_3R<0x385a0000>;
+def AMCAS__DB_H  : AMCAS_3R<0x385a8000>;
+def AMCAS__DB_W  : AMCAS_3R<0x385b0000>;
+def AMCAS__DB_D  : AMCAS_3R<0x385b8000>;
 def LL_D : LLBase<0x22000000>;
 def SC_D : SCBase<0x23000000>;
 def SC_Q : SCBase_128<0x38570000>;