[docs] Make WritingAnLLVMPass.rst up-to-date with current state of things

Description

[docs] Make WritingAnLLVMPass.rst up-to-date with current state of things

This patch updates WritingAnLLVMPass.rst to make it in line with current state of things.

Specifically:

  • Makefile instructions replaced with CMake ones
  • Filenames replaced with correct ones
  • Example reformatted a bit to make it less confusing and more conforming to LLVM Coding Standards
  • opt tool output updated with what it actually prints nowdays
  • "gcse" (which doesn't exist anymore) replaced with "gvn" (which still does)

Differential Revision: https://reviews.llvm.org/D24233

Details