[TableGen] When trying to reuse a scheduler class for instructions from an...

[TableGen] When trying to reuse a scheduler class for instructions from an InstRW, make sure we haven't already seen another InstRW containing this instruction on this CPU.

This is similar to the check later when we remap some of the instructions from one class to a new one. But if we reuse the class we don't get to do that check.

So many CPUs have violations of this check that I had to add a flag to the SchedMachineModel to allow it to be disabled. Hopefully we can get those cleaned up quickly and remove this flag.

A lot of the violations are due to overlapping regular expressions, but that's not the only kind of issue it found.

llvm-svn: 327808