Improve C1 register allocator logging and debugging support

In the process of understanding and tracing back the problem of JDK-8249603, I used some additional debugging and logging code. In this RFE, I want to add this code and some additional enhancements to simplify the debugging of another C1 register allocator bug in the future.

Summary:
- find_interval(number): Can be called like that from gdb anywhere to find an interval with the given number.
- Interval::print_children()/print_parent(): Useful when debugging with gdb to quickly show the split children and parent.
- LinearScan::print_reg_num(number): Prints the register or stack location for this register number. This is useful in some places (logging with TraceLinearScanLevel set) where it just printed a number which first had to be manually looked up in other logs.

Possible future work (not covered in this RFE):
- The TraceLinearScanLevel flag is too generic and prints a lot of things (especially at level 4). It would be useful to split it into separate flags to only print a subset of the entire linear scan register allocator algorithm or a specific part but with more details.