[Lexicon] Add "GEP"

Summary:
`getelementptr` is frequently abbreviated as "GEP", often in source files that
do not ever reference the full name of the instruction. Add it to the Lexicon,
in case readers go to look for what it means there.

Test plan:
1. `ninja sphinx`
2. Confirm that the rendered docs HTML contains the new "GEP" entry

llvm-svn: 311168

llvm/docs/Lexicon.rst

@@ -109,6 +109,11 @@ G
     Garbage Collection. The practice of using reachability analysis instead of
     explicit memory management to reclaim unused memory.
 
+**GEP**
+    ``GetElementPtr``. An LLVM IR instruction that is used to get the address
+    of a subelement of an aggregate data structure. It is documented in detail
+    `here <http://llvm.org/docs/GetElementPtr.html>`_.
+
 **GVN**
     Global Value Numbering. GVN is a pass that partitions values computed by a
     function into congruence classes. Values ending up in the same congruence