cmd/compile: remove funcdepth variables

There were only two large classes of use for these variables:

1) Testing "funcdepth != 0" or "funcdepth > 0", which is equivalent to
checking "Curfn != nil".

2) In oldname, detecting whether a closure variable has been created
for the current function, which can be handled by instead testing
"n.Name.Curfn != Curfn".

Lastly, merge funcstart into funchdr, since it's only called once, and
it better matches up with funcbody now.

Passes toolstash-check.

Change-Id: I8fe159a9d37ef7debc4cd310354cea22a8b23394
Reviewed-on: https://go-review.googlesource.com/99076
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/pgen.go

@@ -204,13 +204,11 @@ func funccompile(fn *Node) {
 	}
 
 	dclcontext = PAUTO
-	funcdepth = fn.Func.Depth + 1
 	Curfn = fn
 
 	compile(fn)
 
 	Curfn = nil
-	funcdepth = 0
 	dclcontext = PEXTERN
 }