Use expandCodeFor instead of expand when the result will be

Use expandCodeFor instead of expand when the result will be
immediately casted. At present, this is just a minor code
simplification. In the future, the expansion code may be able
to make better choices if it knows what the desired result
type will be.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73137 91177308-0d34-0410-b5e6-96231b3b80d8