[PowerPC] [Constant Hoisting] Enable constant hoisting on PPC

Implements the various TTI functions to enable constant hoisting on PPC. The
only significant test-suite change is this:

MultiSource/Benchmarks/VersaBench/bmm/bmm - 20% speedup
(which essentially reverses the slowdown from r206120).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206141 91177308-0d34-0410-b5e6-96231b3b80d8