OpcodeDispatcher: Use lshr for ah/bh with AllowUpperGarbage

If we ever get around to fusing ops with shifts in the ConstProp optimizer (may
or may not be worthwhile), this will delete an instruction from things like "or
al, bh".

Even though lsr is the same speed as bfe on Firestorm, I feel if you ask for
garbage you should get garbage C:

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>

FEXCore/Source/Interface/Core/OpcodeDispatcher.cpp

@@ -5167,7 +5167,11 @@ OrderedNode *OpDispatchBuilder::LoadGPRRegister(uint32_t GPR, int8_t Size, uint8
 
   if ((!AllowUpperGarbage && (Size != GPRSize)) || Offset != 0) {
     // Extract the subregister if requested.
-    Reg = _Bfe(IR::SizeToOpSize(std::max<uint8_t>(4u, Size)), Size * 8, Offset, Reg);
+    const auto OpSize = IR::SizeToOpSize(std::max<uint8_t>(4u, Size));
+    if (AllowUpperGarbage)
+      Reg = _Lshr(OpSize, Reg, _Constant(Offset));
+    else
+      Reg = _Bfe(OpSize, Size * 8, Offset, Reg);
   }
   return Reg;
 }