[dev.regabi] cmd/compile: cleanup export code further
This CL rips off a number of toolstash bandages:
- Fixes position information for string concatenation.
- Adds position information for struct literal fields.
- Removes unnecessary exprsOrNil calls or replaces them with plain
expr calls when possible.
- Reorders conversion expressions to put type first, which matches
source order and also the order the importer needs for calling the
ConvExpr constructor.
Change-Id: I44cdc6035540d9ecefd9c1bcd92b8711d6ed813c
Reviewed-on: https://go-review.googlesource.com/c/go/+/279957
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>