Make inferLibFuncAttributes() add SExt attribute on second arg to ldexp.

This was missing as discovered by the SystemZ multistage bot:
http://lab.llvm.org:8011/#/builders/8, where wrong code resulted when this
extension was not performed.

Thanks for review by Ulrich Weigand and Roman Lebedev.

Differential Revision: https://reviews.llvm.org/D90760