Use local assertion prop to omit casts on returns (#55632)

When inserting normalizing casts for returns in pre-order
morph, "fgMorphCast" was used, which did not account for
the fact that local assertion propagation could tell us
that the cast is in fact unnecessary. Fix this by calling
"fgMorphTree" instead.

Also some miscellaneous code cleanup.

src/coreclr/jit/morph.cpp

@@ -11482,7 +11482,7 @@ GenTree* Compiler::fgMorphSmpOp(GenTree* tree, MorphAddrContext* mac)
 
         case GT_RETURN:
             // normalize small integer return values
-            if (fgGlobalMorph && varTypeIsSmall(info.compRetType) && (op1 != nullptr) && (op1->TypeGet() != TYP_VOID) &&
+            if (fgGlobalMorph && varTypeIsSmall(info.compRetType) && (op1 != nullptr) && !op1->TypeIs(TYP_VOID) &&
                 fgCastNeeded(op1, info.compRetType))
             {
                 // Small-typed return values are normalized by the callee
@@ -11491,11 +11491,10 @@ GenTree* Compiler::fgMorphSmpOp(GenTree* tree, MorphAddrContext* mac)
                 // Propagate GTF_COLON_COND
                 op1->gtFlags |= (tree->gtFlags & GTF_COLON_COND);
 
-                tree->AsOp()->gtOp1 = fgMorphCast(op1);
+                tree->AsOp()->gtOp1 = fgMorphTree(op1);
 
                 // Propagate side effect flags
-                tree->gtFlags &= ~GTF_ALL_EFFECT;
-                tree->gtFlags |= (tree->AsOp()->gtOp1->gtFlags & GTF_ALL_EFFECT);
+                tree->SetAllEffectsFlags(tree->AsOp()->gtGetOp1());
 
                 return tree;
             }