cmd/compile/internal/noder: refactor exprConvert code

cmd/compile/internal/noder: refactor exprConvert code

This CL deduplicates the explicit and implicit exprConvert code paths
to have a single common function, so they're easier to keep in sync.

Change-Id: I2b145d2ce6de6018ffc2db5cdb9d891f4e223381
Reviewed-on: https://go-review.googlesource.com/c/go/+/427677
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>