cmd/compile: allow OpArgXXXReg comes before LoweredGetClosurePtr

Both OpArgXXXReg and LoweredGetClosurePtr must come very early,
because they carry registers that are technically live on entry.
But no need to impose ordering requirement between them.

Change-Id: Iee1db6239a75e5b381e0ad25ba5503169333217b
Reviewed-on: https://go-review.googlesource.com/c/go/+/309629


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>