[docs][LoopTerminology] Add Loop Simplify Form description.

Information taken from https://youtu.be/3pRhvQi7Z10?t=481 and
comments in LoopSimplify.h.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D74989

Author: baziotis

llvm/docs/LoopTerminology.rst

@@ -141,10 +141,25 @@ are important for working successfully with this interface.
   reachability of the loop.
 
 
+.. _loop-terminology-loop-simplify:
+
 Loop Simplify Form
 ==================
 
-TBD
+The Loop Simplify Form is a canonical form that makes
+several analyses and transformations simpler and more effective.
+It is ensured by the LoopSimplify
+(:ref:`-loop-simplify <passes-loop-simplify>`) pass and is automatically
+added by the pass managers when scheduling a LoopPass.
+This pass is implemented in
+`LoopInfo.h <http://llvm.org/doxygen/LoopSimplify_8h_source.html>`_.
+When it is successful, the loop has:
+
+* A preheader.
+* A single backedge (which implies that there is a single latch).
+* Dedicated exits. That is, no exit block for the loop
+  has a predecessor that is outside the loop. This implies
+  that all exit blocks are dominated by the loop header.
 
 
 Loop Closed SSA (LCSSA)

llvm/docs/Passes.rst

@@ -803,12 +803,15 @@ loop to increment the value by the appropriate amount.
 
 A simple loop rotation transformation.
 
+.. _passes-loop-simplify:
+
 ``-loop-simplify``: Canonicalize natural loops
 ----------------------------------------------
 
 This pass performs several transformations to transform natural loops into a
 simpler form, which makes subsequent analyses and transformations simpler and
-more effective.
+more effective. A summary of it can be found in
+:ref:`Loop Terminology, Loop Simplify Form <loop-terminology-loop-simplify>`.
 
 Loop pre-header insertion guarantees that there is a single, non-critical entry
 edge from outside of the loop to the loop header.  This simplifies a number of