[Fixed Point] Move the compassign LHS type correction a bit further down. NFCI.

[ebevhan]

Summary

We can simplify the LHSTy correction for
fixed-point compassign by moving it below
the point where we know we have a compound
assignment.

Also, we shouldn't look at the LHS and RHS
separately; look at the computation result
type instead.

Looking at the LHS and RHS is also wrong
for compassigns with fixed and floating
point (though this does not work upstream
yet).

[leonardchan]

LGTM

clang/lib/Sema/SemaExpr.cpp

if (getLangOpts().CPlusPlus && !RHS.isInvalid()) {
       VK = RHS.get()->getValueKind();
       OK = RHS.get()->getObjectKind();
     }
     break;
   }
   if (ResultTy.isNull() || LHS.isInvalid() || RHS.isInvalid())
     return ExprError();
 
-  // The LHS is not converted to the result type for fixed-point compound
-  // assignment as the common type is computed on demand. Reset the CompLHSTy
-  // to the LHS type we would have gotten after unary conversions.
-  if (!CompLHSTy.isNull() &&
-      (LHS.get()->getType()->isFixedPointType() ||
-       RHS.get()->getType()->isFixedPointType()))
-    CompLHSTy = UsualUnaryConversions(LHS.get()).get()->getType();
-
   if (ResultTy->isRealFloatingType() &&
       (getLangOpts().getFPRoundingMode() != RoundingMode::NearestTiesToEven ||
        getLangOpts().getFPExceptionMode() != LangOptions::FPE_Ignore))
     // Mark the current function as usng floating point constrained intrinsics
     if (FunctionDecl *F = dyn_cast<FunctionDecl>(CurContext)) {
       F->setUsesFPIntrin(true);
     }
 
ExprResult Sema::CreateBuiltinBinOp(SourceLocation OpLoc,
 
   // Handle compound assignments.
   if (getLangOpts().CPlusPlus && LHS.get()->getObjectKind() !=
       OK_ObjCProperty) {
     VK = VK_LValue;
     OK = LHS.get()->getObjectKind();
   }
 
+  // The LHS is not converted to the result type for fixed-point compound
+  // assignment as the common type is computed on demand. Reset the CompLHSTy
+  // to the LHS type we would have gotten after unary conversions.
+  if (CompResultTy->isFixedPointType())
+    CompLHSTy = UsualUnaryConversions(LHS.get()).get()->getType();
+
   if (ConvertHalfVec)
     return convertHalfVecBinOp(*this, LHS, RHS, Opc, ResultTy, VK, OK, true,
                                OpLoc, CurFPFeatures);
 
   return CompoundAssignOperator::Create(Context, LHS.get(), RHS.get(), Opc,
                                         ResultTy, VK, OK, OpLoc, CurFPFeatures,
                                         CompLHSTy, CompResultTy);
 }