cmd/compile: remove no-longer-necessary recursive inlining checks

cmd/compile: remove no-longer-necessary recursive inlining checks

this does result in a little bit more inlining,
cmd/compile text is 0.5% larger,
bent-benchmark text geomeans grow by only 0.02%.
some of our tests make assumptions about inlining.

Change-Id: I999d1798aca5dc64a1928bd434258a61e702951a
Reviewed-on: https://go-review.googlesource.com/c/go/+/655157
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>