[Hexagon] Return an i64 for result 0 from LowerREADCYCLECOUNTER instead of an i32.

As far as I can tell, the node coming in has an i64 result so the
return should have the same type. The HexagonISD node used for
this has a type profile that says the result is i64.

Found while trying to add assserts to LegalizeDAG to catch
result type mismatches.

Reviewed By: kparzysz

Differential Revision: https://reviews.llvm.org/D98962