Rename ConvergingScheduler to GenericScheduler.

This was an experimental scheduler a year ago. It's now used by
several subtargets, both in-order and out-of-order, and it
is about to be enabled by default for x86 and armv7. It will be the
new GenericScheduler for subtargets that don't provide their own
SchedulingStrategy.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191051 91177308-0d34-0410-b5e6-96231b3b80d8