Clarify uniqueness vs nesting rules

Author: JosephTremoulet

src/jit/optimizer.cpp

@@ -1428,8 +1428,7 @@ namespace
 // We will use the following terminology:
 //   HEAD    - the basic block that flows into the loop ENTRY block (Currently MUST be lexically before entry).
 //             Not part of the looping of the loop.
-//   FIRST   - the lexically first basic block (in bbNext order) within this loop.  (May be part of a nested loop,
-//             but not the outer loop. ???)
+//   FIRST   - the lexically first basic block (in bbNext order) within this loop.
 //   TOP     - the target of the backward edge from BOTTOM. In most cases FIRST and TOP are the same.
 //   BOTTOM  - the lexically last block in the loop (i.e. the block from which we jump to the top)
 //   EXIT    - the predecessor of loop's unique exit edge, if it has a unique exit edge; else nullptr
@@ -1439,12 +1438,16 @@ namespace
 //   When the loop is identified, blocks will be moved out to make it a compact contiguous region if possible,
 //   and in cases where compaction is not possible, we'll subsequently treat all blocks in the lexical range
 //   between TOP and BOTTOM as part of the loop even if they aren't part of the SCC.
+//   Regarding nesting:  Since a given block can only have one back-edge (we only detect loops with back-edges
+//   from BBJ_COND or BBJ_ALWAYS blocks), no two loops will share the same BOTTOM.  Two loops may share the
+//   same FIRST/TOP/ENTRY as reported by LoopSearch, and optCanonicalizeLoopNest will subsequently re-write
+//   the CFG so that no two loops share the same FIRST/TOP/ENTRY anymore.
 //
 //        |
 //        v
 //      head
 //        |
-//        |    top/beg <--+
+//        |  top/first <--+
 //        |       |       |
 //        |      ...      |
 //        |       |       |