[release-branch.go1.11] cmd/compile: don't deadcode eliminate labels

[release-branch.go1.11] cmd/compile: don't deadcode eliminate labels

Dead-code eliminating labels is tricky because there might
be gotos that can still reach them.

Bug probably introduced with CL 91056

Fixes #28617

Change-Id: I6680465134e3486dcb658896f5172606cc51b104
Reviewed-on: https://go-review.googlesource.com/c/147817
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Iskander Sharipov <iskander.sharipov@intel.com>
Reviewed-on: https://go-review.googlesource.com/c/147857