cmd/compile: avoid slicebytetostring call in len(string([]byte))

cmd/compile: avoid slicebytetostring call in len(string([]byte))

Change-Id: Ie04503e61400a793a6a29a4b58795254deabe472
Reviewed-on: https://go-review.googlesource.com/c/go/+/497276
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>