Jit64: subfx - Use LEA when possible

Similar to what we do for addx. Since we're calculating b - a and
because subtraction is not communitative, we can only apply this when
source register a holds the constant.

Before:
45 8B EE             mov         r13d,r14d
41 83 ED 08          sub         r13d,8

After:
45 8D 6E F8          lea         r13d,[r14-8]

Source/Core/Core/PowerPC/Jit64/Jit_Integer.cpp

@@ -932,6 +932,10 @@ void Jit64::subfx(UGeckoInstruction inst)
       MOV(32, Rd, Rb);
       SUB(32, Rd, R(RSCRATCH));
     }
+    else if (Rb.IsSimpleReg() && Ra.IsImm() && !inst.OE)
+    {
+      LEA(32, Rd, MDisp(Rb.GetSimpleReg(), -Ra.SImm32()));
+    }
     else
     {
       MOV(32, Rd, Rb);