[CodeGenPrepare] Refine the cost model provided by the promotion helper.

[CodeGenPrepare] Refine the cost model provided by the promotion helper.
- Use TargetLowering to check for the actual cost of each extension.
- Provide a factorized method to check for the cost of an extension:
  TargetLowering::isExtFree.
- Provide a virtual method TargetLowering::isExtFreeImpl for targets to be able
  to tune the cost of non-free extensions.

This refactoring offers a better granularity to model what really happens on
different targets.

No performance changes and very few code differences.

Part of <rdar://problem/19267165>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231855 91177308-0d34-0410-b5e6-96231b3b80d8