[wasm] Retype integer constant struct returns to the native return type

When LowerRetStruct retypes a struct return to a primitive nativeReturnType and
the return value is a GT_CNS_INT, the integer branch left the constant as TYP_INT
even when the native return type is wider (e.g. TYP_LONG). On register targets
this is harmless because integer registers are untyped, but wasm has a typed
value stack, so codegen emitted i32.const 0 for an i64-typed return.

This surfaced validating the R2R-compiled CoreLib for wasm:
Vector64<byte>.get_Zero returns a zero-initialized 8-byte struct, which the wasm
ABI lowers to an i64 return. The emitted body ended with i32.const 0 and wasmtime
rejected the module with "type mismatch: expected i64, found i32".

Retype the integer constant to the native return type under TARGET_WASM so
codegen emits an i64.const. Applies to both browser and wasi, which share the JIT.

Also submitted standalone as dotnet#129102; will reconcile if that lands
first.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lewing

src/coreclr/jit/lower.cpp

@@ -5859,6 +5859,16 @@ void Lowering::LowerRetStruct(GenTreeUnOp* ret)
             else
             {
                 assert(varTypeUsesIntReg(nativeReturnType));
+#ifdef TARGET_WASM
+                // Wasm has a typed value stack, so the constant's type must match the
+                // type the function returns. Retype an INT constant (e.g. a zero from a
+                // promoted/zero-initialized struct field) to the wider native return
+                // type so codegen emits an i64.const rather than an i32.const.
+                if (genActualType(retVal) != genActualType(nativeReturnType))
+                {
+                    retVal->ChangeType(genActualType(nativeReturnType));
+                }
+#endif // TARGET_WASM
             }
             break;
         }