Do not eliminate casts from FP types

An optimization in morph was deleting casts
on the RHS of a narrow store if the cast-to-type
was wider than the type being stored. This is only
valid for casts from integral types, as the backend
does not handle "STOREIND(byte*, double)", nor is there
an instruction to go from an XMM register to a narrow
memory location on x86/x64.

The issue is not reproducible right now because
fgMorphCast wraps the casts in question, but it is
an invalid IR transformation nonetheless, and similar
code in fgMorphSmpOpOptional guards against non-integral sources.

src/coreclr/jit/morph.cpp

@@ -13378,18 +13378,19 @@ GenTree* Compiler::fgMorphSmpOp(GenTree* tree, MorphAddrContext* mac)
                 tree->AsOp()->gtOp1 = op1;
             }
 
-            /* If we are storing a small type, we might be able to omit a cast */
-            if ((effectiveOp1->gtOper == GT_IND) && varTypeIsSmall(effectiveOp1->TypeGet()))
+            // If we are storing a small type, we might be able to omit a cast.
+            if (effectiveOp1->OperIs(GT_IND) && varTypeIsSmall(effectiveOp1))
             {
-                if (!gtIsActiveCSE_Candidate(op2) && (op2->gtOper == GT_CAST) && !op2->gtOverflow())
+                if (!gtIsActiveCSE_Candidate(op2) && op2->OperIs(GT_CAST) &&
+                    varTypeIsIntegral(op2->AsCast()->CastOp()) && !op2->gtOverflow())
                 {
                     var_types castType = op2->CastToType();
 
                     // If we are performing a narrowing cast and
                     // castType is larger or the same as op1's type
                     // then we can discard the cast.
 
-                    if (varTypeIsSmall(castType) && (genTypeSize(castType) >= genTypeSize(effectiveOp1->TypeGet())))
+                    if (varTypeIsSmall(castType) && (genTypeSize(castType) >= genTypeSize(effectiveOp1)))
                     {
                         tree->AsOp()->gtOp2 = op2 = op2->AsCast()->CastOp();
                     }