update_test_checks: keep meta variables stable by default

Prior to this change, running UTC on larger tests, especially tests
with unnamed IR values, often resulted in a spuriously large diff
because e.g. TMPnn variables in the CHECK lines were renumbered. This
change attempts to reduce the diff by keeping those variable names the
same.

There are cases in which this "drift" of variable names can end up being
more confusing. The old behavior can be re-enabled with the
--reset-variable-names command line argument.

The improvement may not be immediately apparent in the diff of this change.
The point is that the diff of stable_ir_values.ll against
stable_ir_values.ll.expected after this change is smaller.

Ideally, we'd also keep meta variables for "global" objects stable, e.g.
for attributes (#nn) and metadata (!nn). However, that would require a
much more substantial refactoring of how we generate check lines, so I
left it for future work.