EXTRACT_SUBREG coalescing support. The coalescer now treats EXTRACT_SUBREG like

(almost) a register copy. However, it always coalesced to the register of the
RHS (the super-register). All uses of the result of a EXTRACT_SUBREG are sub-
register uses which adds subtle complications to load folding, spiller rewrite,
etc.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42899 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/ScheduleDAG.cpp

@@ -567,9 +567,6 @@ void ScheduleDAG::EmitSubregNode(SDNode *Node,
     // TODO: If the node is a use of a CopyFromReg from a physical register
     // fold the extract into the copy now
 
-    // TODO: Add tracking info to SSARegMap of which vregs are subregs
-    // to allow coalescing in the allocator
-    
     // Create the extract_subreg machine instruction.
     MachineInstr *MI =
       new MachineInstr(BB, TII->get(TargetInstrInfo::EXTRACT_SUBREG));