cmd/compile: schedule OpArg earlier in blocks for better debugging
cmd/compile: schedule OpArg earlier in blocks for better debugging

The location list for OpArg starts where the OpArg appears;
this is not necessarily as soon as the OpArg coulde be
observed, and it is reasonable for a user to expect that
if a breakpoint is set "on f" then the arguments to f will
be observable where that breakpoint happens to be set (this
may also require setting the breakpoint after the prologue,
but that is another issue).

Change-Id: I0a1b848e50f475e5d8a5fad781241126872a0400
Reviewed-on: https://go-review.googlesource.com/c/142819
Run-TryBot: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Reviewed-by: Alessandro Arzilli <e26d9aa77fb315046750e4f7a9c5cd199e338f92@gmail.com>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

golang/go,golang/go,CAFxX/go,golang/go,golang/go,CAFxX/go,CAFxX/go,josharian/go,golang/go,cloudflare/go,josharian/go,akutz/go,CAFxX/go,cloudflare/go,josharian/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,josharian/go,akutz/go,josharian/go,cloudflare/go,cloudflare/go,nawawi/go,josharian/go,cloudflare/go,nawawi/go,nawawi/go,akutz/go,nawawi/go,akutz/go,golang/go,akutz/go,josharian/go,josharian/go,cloudflare/go,akutz/go,akutz/go,akutz/go,nawawi/go,golang/go,CAFxX/go,golang/go,nawawi/go