[SystemZ] Fix argument type of tdc instruction.

The DAG node for the Test Data Class is defined using i64 as the second parameter.
However, the code to lower is_fpclass uses `i32` as type. This only works because no
type check is generated in the DAG matcher.
This PR changes the type of the mask constant to `i64`.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D125230