Make NumMicroOps a variable in the subtarget's instruction itinerary.

Make NumMicroOps a variable in the subtarget's instruction itinerary.

The TargetInstrInfo::getNumMicroOps API does not change, but soon it
will be used by MachineScheduler. Now each subtarget can specify the
number of micro-ops per itinerary class. For ARM, this is currently
always dynamic (-1), because it is used for load/store multiple which
depends on the number of register operands.

Zero is now a valid number of micro-ops. This can be used for
nop pseudo-instructions or instructions that the hardware can squash
during dispatch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159406 91177308-0d34-0410-b5e6-96231b3b80d8