Update SPIRVRepresentationInLLVM.rst to include loop controls (#2592)

Add section about loop controls

docs/SPIRVRepresentationInLLVM.rst

@@ -184,7 +184,7 @@ The unmangled names of SPIR-V builtin BuildNDRange functions follow the conventi
   __spirv_{BuildNDRange}_{1|2|3}D
 
 SPIR-V 1.1 Builtin CreatePipeFromPipeStorage Function Name
-----------------------------------------
+----------------------------------------------------------
 
 The unmangled names of SPIR-V builtin CreatePipeFromPipeStorage function follow the convention:
 
@@ -213,7 +213,7 @@ starts with two underscores to facilitate identification since extended instruct
 may contain underscore. The remaining postfixes start with one underscore.
 
 OpenCL Extended Builtin Vector Load Function Names
-----------------------------------------
+--------------------------------------------------
 
 The unmangled names of OpenCL extended vector load functions follow the convention:
 
@@ -422,7 +422,7 @@ are translated for image types, but they should be encoded in LLVM IR type name
 rather than function metadata.
 
 Function parameter, instruction and global variable decoration through metadata
-------------------------------------------------------------------
+-------------------------------------------------------------------------------
 
 Function parameters, instructions and global variables can be decorated using LLVM
 metadata through the metadata names ``spirv.ParameterDecorations`` and
@@ -483,6 +483,26 @@ extra operands ``i32 1`` and ``i32 2``.
 decorates the argument ``b`` of ``k`` with ``Restrict`` in SPIR-V while not
 adding any decoration to argument ``a``.
 
+Loop controls and loop metadata
+-------------------------------
+
+SPIR-V Loop controls that do not have corresponding `llvm.loop` metadata
+can be decorated using LLVM metadata through the names
+``spirv.loop.<LoopControlID>`` where ``LoopControlID`` corresponds to the name
+of the SPIR-V loop control. This metadata should be applied to the latch-block's
+branch instruction.
+
+An example with the ``DependencyAccessesINTEL`` loop control:
+
+.. code-block:: llvm
+  
+  br i1 %cond, label %loop, label %exit !spirv.loop.dependency_accesses !4
+  ...
+  !1 = distinct !{}      ; metadata corresponding to distinct access group
+  !2 = distinct !{}      ; metadata corresponding to distinct access group
+  !3 = distinct !{}      ; metadata corresponding to distinct access group
+  !4 = !{!0, !1, !2, 0}  ; metadata node grouping access groups for the corresponding DepenencyAccessesINTEL instance
+
 Member decoration through pointer annotations
 ---------------------------------------------