Revert "[ARM][MVE] Add intrinsics for immediate shifts."

and two follow-on commits: one warning fix and one functionality.

As it's breaking at least the lto bot:

http://lab.llvm.org:8011/builders/clang-with-lto-ubuntu/builds/15132/steps/test-stage1-compiler/logs/stdio

This reverts commits:

 8d70f3c9
 ff4dceef
 d97b3e3e