[dev.unified] cmd/compile/internal/noder: add optExpr for optional expressions
Previously, {writer,reader}.expr would allow for nil
expressions (i.e., no expression at all, not a "nil" identifier). But
only a few contexts allow this, and it simplifies some logic if we can
assume the expression is non-nil.
So this CL introduces optExpr as a wrapper method for handling nil
expressions specially.
Change-Id: I438bae7a3191126f7790ec0bf5b77320fe855514
Reviewed-on: https://go-review.googlesource.com/c/go/+/410099
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>