utils/update_mir_test_checks.py: allow checking fixedStack in .mir files

Generation of CHECK lines for fixedStack can be enabled with --print-fixed-stack.
This is particularly useful for tests which need to inspect how the
stack looks, e.g. for ABI tests.

See the other stacked revision building on top of this one which enables UTC_ARGS (in a similar fashion to other test updaters in utils/).

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D135579