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/sizeof_test.go

@@ -22,8 +22,8 @@ func TestSizeof(t *testing.T) {
 		_32bit uintptr     // size on 32bit platforms
 		_64bit uintptr     // size on 64bit platforms
 	}{
-		{Func{}, 132, 232},
-		{Name{}, 36, 56},
+		{Func{}, 128, 232},
+		{Name{}, 32, 56},
 		{Param{}, 28, 56},
 		{Node{}, 76, 128},
 	}