cmd/compile: stop changing Field.Sym for parameters

cmd/compile: stop changing Field.Sym for parameters

Field.Sym now always contains the original symbol as it appeared in Go
source, so we don't need OrigSym anymore.

Instead, when the mangled name is desired, Field.Nname.Sym() can be
used instead, which is always non-nil if Nname is non-nil.

Change-Id: I96cd61db6458d4a2e07ec5810239236e3dfba747
Reviewed-on: https://go-review.googlesource.com/c/go/+/527516
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>