[update_mir_test_checks.py] Use -NEXT FileCheck directories

Previously the script emitted output using plain CHECK directives. This
can result in a test passing even if there are some instructions between
CHECK directives that should have been removed. It also makes debugging
tests that have the output in a different order more difficult since
FileCheck can match with a later line and then complain about the "wrong"
directive not being found.

This will cause quite large diffs when updating existing tests, but I'm not sure we need an opt-in flag here.

Depends on D109765 (pre-commit tests)

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D109767