[Docs] Update PatFrag definition in documentation

[luke]

Summary

The old definition for store was outdated

[asb]

llvm/docs/WritingAnLLVMBackend.rst

1314

How about "the DAG operator for store is defined below (alongside the definitions it depends on):" so it's less confusing why we're listing st and unindexedstore as well?

I note that the previous version had st without including it's definition, I'm in two minds as to whether it's worth including it (as you do in your patch) or not...

[luke]

Remove dependent definitions, keep it succinct

[asb]

LGTM, thanks!

llvm/docs/WritingAnLLVMBackend.rst

 .. code-block:: text
 
   def ADDRrr : ComplexPattern<i32, 2, "SelectADDRrr", [], []>;
 
 The definition of ``ADDRrr`` refers to ``SelectADDRrr``, which is a function
 defined in an implementation of the Instructor Selector (such as
 ``SparcISelDAGToDAG.cpp``).
 
 In ``lib/Target/TargetSelectionDAG.td``, the DAG operator for store is defined
 below:
 
 .. code-block:: text
 
   def store : PatFrag<(ops node:$val, node:$ptr),
-                      (st node:$val, node:$ptr), [{
-    if (StoreSDNode *ST = dyn_cast<StoreSDNode>(N))
-      return !ST->isTruncatingStore() &&
-             ST->getAddressingMode() == ISD::UNINDEXED;
-    return false;
-  }]>;
+                      (unindexedstore node:$val, node:$ptr)> {
+    let IsStore = true;
+    let IsTruncStore = false;
+  }
 
 ``XXXInstrInfo.td`` also generates (in ``XXXGenDAGISel.inc``) the
 ``SelectCode`` method that is used to call the appropriate processing method
 for an instruction.  In this example, ``SelectCode`` calls ``Select_ISD_STORE``
 for the ``ISD::STORE`` opcode.
 
 .. code-block:: c++