LiveIntervalAnalysis: Cleanup computeDeadValues

- This also fixes a bug introduced in r223880 where values were not
  correctly marked as Dead anymore.
- Cleanup computeDeadValues(): split up SubRange code variant, simplify
  arguments.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224538 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/LiveInterval.h

@@ -472,6 +472,12 @@ namespace llvm {
       removeSegment(S.start, S.end, RemoveDeadValNo);
     }
 
+    /// Remove segment pointed to by iterator @p I from this range.  This does
+    /// not remove dead value numbers.
+    iterator removeSegment(iterator I) {
+      return segments.erase(I);
+    }
+
     /// Query Liveness at Idx.
     /// The sub-instruction slot of Idx doesn't matter, only the instruction
     /// it refers to is considered.