[dev.regabi] cmd/compile: always use the compile queue

The compiler currently has two modes for compilation: one where it
compiles each function as it sees them, and another where it enqueues
them all into a work queue. A subsequent CL is going to reorder
function compilation to ensure that functions are always compiled
before any non-trivial function literals they enclose, and this will
be easier if we always use the compile work queue.

Also, fewer compilation modes makes things simpler to reason about.

Change-Id: Ie090e81f7476c49486296f2b90911fa0a466a5dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/283313


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>