Changes

[InstSimplify] Update comments, remove redundant tests

[LTO] Remove options to disable inlining, vectorization & GVNLoadPRE.

This patch removes some ancient options as a clean-up before moving
code-gen to use LTOBackend in D94487.

I think it would preferable to remove those ancient options, because

  1. There are no corresponding options in LTOBackend based tools,
  2. There are no unit tests for them,
  3. They are not passed through by Clang,
  4. At least for GNVLoadPRE, users could just use GVN's `enable-load-pre`.

Alternatively we could add support for those options to lto::Config &
co, but I think it would be better to remove them, unless they are
actually used in practice.

Reviewed By: steven_wu, tejohnson

Differential Revision: https://reviews.llvm.org/D94783