Instead of always inserting a nop to use as the target of an inline
mark, see if we can instead find an instruction we're issuing anyway
with the correct line number, and use that instruction. That way, we
don't need to issue a nop.

Makes cmd/go 0.3% smaller.

Update #29571

Change-Id: If6cfc93ab3352ec2c6e0878f8074a3bf0786b2f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/158021
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>