[PowerPC] Add PowerPC population count, reversed load and store related...

[PowerPC] Add PowerPC population count, reversed load and store related builtins and instrinsics for XL compatibility

This patch is in a series of patches to provide builtins for compatibility
with the XL compiler. This patch adds the builtins and instrisics for population
count, reversed load and store related operations.

Reviewed By: nemanjai, #powerpc

Differential revision: https://reviews.llvm.org/D106021