[PowerPC] Materialize 34 bit constants with pli on Power 10.

NOTE: This patch was originally written by Anil Mahmud. His code has been
rebased but otherwise left mostly unchanged.

A new instructon on Power 10 allows for the materialization of 34 bit
immediate values. This patch allows the compiler to take advantage of
the new instruction in this situation.

Reviewed By: amyk

Differential Revision: https://reviews.llvm.org/D92879