shader/arithmetic_integer: Implement IADD.X

IADD.X takes the carry flag and adds it to the result. This is generally
used to emulate 64-bit operations with 32-bit registers.

src/video_core/engines/shader_bytecode.h

@@ -813,6 +813,10 @@ union Instruction {
         BitField<49, 1, u64> negate_a;
     } alu_integer;
 
+    union {
+        BitField<43, 1, u64> x;
+    } iadd;
+
     union {
         BitField<39, 1, u64> ftz;
         BitField<32, 1, u64> saturate;