TableGen: Add -gen-register-info-debug-dump

Adds a new --gen-register-info-debug-dump mode to tablegen that dumps various register related information:

- List of register classes with super and subclasses
- List of subregister indexes with lanemasks
- List of registers with subregisters

I will use this in an upcoming commit to create a test.

It may also be useful for target developers wanting to get an overview
of all the register related information, esp. the things inferred by
tablegen and not directly visible in the .td file.

Differential Revision: https://reviews.llvm.org/D36911

llvm-svn: 311913