JitArm64: Add special zero case to ADDI2R

This normally doesn't reduce the instruction count, but is nonetheless
useful on CPUs that can do 0-cycle moves.

Source/Core/Common/Arm64Emitter.cpp

@@ -4125,14 +4125,24 @@ void ARM64XEmitter::AddImmediate(ARM64Reg Rd, ARM64Reg Rn, u64 imm, bool shift,
 void ARM64XEmitter::ADDI2R_internal(ARM64Reg Rd, ARM64Reg Rn, u64 imm, bool negative, bool flags,
                                     ARM64Reg scratch)
 {
+  DEBUG_ASSERT(Is64Bit(Rd) == Is64Bit(Rn));
+
   if (!Is64Bit(Rd))
     imm &= 0xFFFFFFFFULL;
 
   bool has_scratch = scratch != ARM64Reg::INVALID_REG;
   u64 imm_neg = Is64Bit(Rd) ? u64(-s64(imm)) : u64(-s64(imm)) & 0xFFFFFFFFuLL;
   bool neg_neg = negative ? false : true;
 
-  // Fast paths, aarch64 immediate instructions
+  // Special path for zeroes
+  if (imm == 0 && !flags)
+  {
+    if (Rd != Rn)
+      MOV(Rd, Rn);
+    return;
+  }
+
+  // Regular fast paths, aarch64 immediate instructions
   // Try them all first
   if (imm <= 0xFFF)
   {