cmd/compile: don't statically copy string-typed variables

During package initialization, the compiler tries to optimize:

    var A = "foo"
    var B = A

into

    var A = "foo"
    var B = "foo"

so that we can statically initialize both A and B and skip emitting
dynamic initialization code to assign "B = A".

However, this isn't safe in the presence of cmd/link's -X flag, which
might overwrite an initialized string-typed variable at link time. In
particular, if cmd/link changes A's static initialization, it won't
know it also needs to change B's static initialization.

To address this, this CL disables this optimization for string-typed
variables.

Fixes #34675.

Change-Id: I1c18f3b855f6d7114aeb39f96aaaf1b452b88236
Reviewed-on: https://go-review.googlesource.com/c/go/+/198657


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>