cmd/compile: performance regression in goawk

[mdempsky]

As reported at https://groups.google.com/g/golang-dev/c/8blU7CnuhZ4/m/d21xuN8hAQAJ, goawk sees a performance slowdown with 1.20 previews.

@randall77 identified that the likely cause is isBigFunc not liking the extra IR generated for a, b = f() style multi-value assignments.

We have logic in the inliner's heuristics already to account for this. I think we just need to repeat the logic in isBigFunc.

[gopherbot]

Change https://go.dev/cl/460395 mentions this issue: cmd/compile/internal/inline: adjust isBigFunc to recognize unified IR codegen