[PowerPC] Add a pass to merge all of the constant global arrays into one pool.

On PowerPC the number of TOC entries must be kept low for large
applications. In order to reduce the number of constant global arrays
we can pool them into one structure and then access them as the base
address of that structure plus some offset. The constant global arrays
may be arrays of `i8` which are constant strings but they may also be
arrays of `i32, i64, etc...`.

Reviewed By: lei, amyk

Differential Revision: https://reviews.llvm.org/D155730