TII: Add documentation about conditional exits. NFC

[iteratee]

Summary

Currently the documented API for AnalyzeBranch leaves no place for
conditional returns. This leaves a blindspot after shrinkwrapping on
platforms with conditional return instructions.

The existing API can be used to handle conditional returns by using a null TBB
and a non-empty condition list. A null TBB should never mean fallthrough with a
non-empty condition list, as that wouldn't make any sense. This leaves a
natural way to represent conditional return branches.

Next up is to add support to PPC for AnalyzeBranch and conditional
return.

[echristo]

So, I'll propose a different suggestion:

TII::BranchInfo {

enum { } // Types of branches.
// Arguments to the function with accessors

}

which encapsulates the existing arguments to make the code a bit more extensible and much easier to read. Then we can extend it for conditional return as a fairly straightforward change.

Sound good?

-eric

include/llvm/Target/TargetInstrInfo.h

   ///    'false' destination in FBB, and a list of operands that evaluate the
   ///    condition.  These operands can be passed to other TargetInstrInfo
   ///    methods to create new branches.
+  /// 5. If this block ends with a conditional ret with fallthrough, it returns
+  ///    null for TBB and FBB and returns a non-empty list of operands that
+  ///    evaluate the condition. These operands can be used to create new
+  ///    branches.
+  /// 6. If the block ends with a conditional return followed by an
+  ///    unconditional branch, the 'false' destination is FBB, while TBB is
+  ///    null. Cond is non-empty as above.
   ///
   /// Note that RemoveBranch and InsertBranch must be implemented to support
   /// cases where this method returns success.