[TTI] Add uniform/non-uniform constant Pow2 detection to...

[TTI] Add uniform/non-uniform constant Pow2 detection to TargetTransformInfo::getInstructionThroughput

This enables us to detect more fast path sdiv cases under cost analysis.

This patch also enables us to handle non-uniform-constant pow2 cases for X86 SDIV costs.

Found while working on D46276

Future patches can then extend the vectorizers to more fully support non-uniform pow2 cases.

Differential Revision: https://reviews.llvm.org/D46637

llvm-svn: 332969