x64Emitter: Avoid 8-bit displacement when possible

Due to the way the ModRM encoding works on x86, memory addressing
combinations involving RBP or R13 need an additional byte for an 8-bit
displacement of zero.

However, this was also applied in cases where it is unnecessary,
effectively wasting a byte.

- MatR with RSP or R12

8B 44 24 00          mov         eax,dword ptr [rsp]
8B 04 24             mov         eax,dword ptr [rsp]

- MRegSum with base != RBP or R13

46 8D 7C 37 00       lea         r15d,[rdi+r14]
46 8D 3C 37          lea         r15d,[rdi+r14]

- MComplex without offset

8B 4C CA 00          mov         ecx,dword ptr [rdx+rcx*8]
8B 0C CA             mov         ecx,dword ptr [rdx+rcx*8]

Source/Core/Common/x64Emitter.cpp

@@ -323,7 +323,11 @@ void OpArg::WriteRest(XEmitter* emit, int extraBytes, X64Reg _operandReg,
       // Okay, we're fine. Just disp encoding.
       // We need displacement. Which size?
       int ioff = (int)(s64)offset;
-      if (ioff < -128 || ioff > 127)
+      if (ioff == 0 && (_offsetOrBaseReg & 7) != 5)
+      {
+        mod = 0;
+      }
+      else if (ioff < -128 || ioff > 127)
       {
         mod = 2;  // 32-bit displacement
       }