cmd/compile: do not mark OpSP, OpSB pos for debugging

Fixes #42801

Change-Id: I2080ecacc109479f5820035401ce2b26d72e2ef2
Reviewed-on: https://go-review.googlesource.com/c/go/+/273506


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>