Revert "cmd/compile: enable zero-copy string->[]byte conversions"

Revert "cmd/compile: enable zero-copy string->[]byte conversions"

This reverts CL 520395.

Reason for revert: thanm@ pointed out failure cases.

Change-Id: I3fd60b73118be3652be2c08b77ab39e793b42110
Reviewed-on: https://go-review.googlesource.com/c/go/+/520596
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>