[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