cmd/compile/internal/inline: adjust isBigFunc to recognize unified IR codegen

cmd/compile/internal/inline: adjust isBigFunc to recognize unified IR codegen

Unified IR generates uniform IR for "a, b = f()" to be able to insert
implicit conversion expressions, but the result is somewhat more
verbose and trips up the inliner's naive cost metrics.

The hairyVisitor.doNode method was already adjusted to account for
this, but isBigFunc needs the same adjustment.

Fixes #57563.

Change-Id: Ia8d86a6e314ec60190c78f40ace4fb30dadc4413
Reviewed-on: https://go-review.googlesource.com/c/go/+/460395
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>