Use update_llc_test_checks for the switch.ll test; add new test

- Change `switch.ll` test to a style suitable for
  `tools/update_llc_test_checks.py`.
- Precommit test for upcoming changes:
  - Add `switch_i8` to `test/CodeGen/X86/switch.ll`.
  - Add `test/CodeGen/X86/switch-phi-const.ll`.

Differential Revision: https://reviews.llvm.org/D124893