cmd/compile: attach slots to incoming params for better debugging

This change attaches a slots to the OpArg values for
incoming params, and this in turn causes location lists
to be generated for params, and that yields better
debugging, in delve and sometimes in gdb.

The parameter lifetimes could start earlier; they are in
fact defined on entry, not at the point where the OpArg is
finally mentioned.  (that will be addressed in another CL)

Change-Id: Icca891e118291d260c35a14acd5bc92bb82d9e9f
Reviewed-on: https://go-review.googlesource.com/c/141697


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>