Merge Tier 1 and 2 interpreters into a single function

[gvanrossum]

Currently the Tier 2 interpreter lives in a separate function. This looks clean but makes switching between tiers complicated. See faster-cpython/ideas#631. So let's make _PyEval_EvalFrameDefault() into a single function that contains both interpreter loops.

Linked PRs

• gh-111520: Integrate the Tier 2 interpreter in the Tier 1 interpreter #111428
• GH-111520: Add back the operand local #111813

[gvanrossum]

FWIW, we're considering to roll this back. See gh-116970.