HLSL: implement 4 (of 6) structuredbuffer types

This is a partial implemention of structurebuffers supporting:

* structured buffer types of:
*   StructuredBuffer
*   RWStructuredBuffer
*   ByteAddressBuffer
*   RWByteAddressBuffer

* Atomic operations on RWByteAddressBuffer

* Load/Load[234], Store/Store[234], GetDimensions methods (where allowed by type)

* globallycoherent flag

But NOT yet supporting:

* AppendStructuredBuffer / ConsumeStructuredBuffer types
* IncrementCounter/DecrementCounter methods

Please note: the stride returned by GetDimensions is as calculated by glslang for std430,
and may not match other environments in all cases.

glslang/Include/intermediate.h

@@ -624,6 +624,15 @@ enum TOperator {
     EOpMethodCalculateLevelOfDetail,     // ...
     EOpMethodCalculateLevelOfDetailUnclamped,     // ...
 
+    // Load already defined above for textures
+    EOpMethodLoad2,                      // Structure buffer object methods.  These are translated to existing
+    EOpMethodLoad3,                      // AST methods, and exist to represent HLSL semantics until that
+    EOpMethodLoad4,                      // translation is performed.  See HlslParseContext::decomposeSampleMethods().
+    EOpMethodStore,                      // ...
+    EOpMethodStore2,                     // ...
+    EOpMethodStore3,                     // ...
+    EOpMethodStore4,                     // ...
+
     // SM5 texture methods
     EOpMethodGatherRed,                  // These are covered under the above EOpMethodSample comment about
     EOpMethodGatherGreen,                // translation to existing AST opcodes.  They exist temporarily