R600: Reorganize tablegen instruction definitions

Each GPU family now has its own file.
SelectionDAG: Allow promotion of SELECT nodes from float to int types

And vice-versa, as long as the types are the same width.

There are a few R600 tests that will cover this.
R600/SI: Promote fp64 SELECT to i64

This type promotion is replacing a Tablegen pattern and it is already
covered by existing tests.