Teach TableGen to evaluate DAG expressions as set operations.

Teach TableGen to evaluate DAG expressions as set operations.

A TableGen backend can define how certain classes can be expanded into
ordered sets of defs, typically by evaluating a specific field in the
record. The SetTheory class can then evaluate DAG expressions that refer
to these named sets.

A number of standard set and list operations are predefined, and the
backend can add more specialized operators if needed. The -print-sets
backend is used by SetTheory.td to provide examples.

This is intended to simplify how register classes are defined:

  def GR32_NOSP : RegisterClass<"X86", [i32], 32, (sub GR32, ESP)>;

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132621 91177308-0d34-0410-b5e6-96231b3b80d8