[X86] Generalize X86PadShortFunction to work with TargetSchedModel

Pre-commit for D45486, don't rely on itinerary scheduler model to determine latencies for padding, use the generic TargetSchedModel::computeInstrLatency call.

Also, replace hard coded (atom specific) 2*uop creation per padding cycle with a version based on the scheduler model's issue width.

Differential Revision: https://reviews.llvm.org/D45486

llvm-svn: 329834