[MachineScheduler] Fix for PR21807: minor code difference building with/without -g.

This patch fixes the issue reported as PR21807. There was a minor difference
in the generated code depending on the -g flag.

The cause was that with -g the machine scheduler used a different
scheduling strategy. This decision was based on the number of instructions
in a schedule region and included debug instructions in that count.

This patch fixes the issue in MISched and provides a test.

Patch by Russell Gallop!