cmd/compile: repair ssa/debug_test regression

Code introduced in

[2034fbab]
cmd/compile: use existing instructions instead of nops for inline marks

to change a src.Pos's column to 1 accidentally reset
the is_stmt and prologue/epilogue bits, and that
turned out to cause a regression in ssa/debug_test.

Preserving that information fixed the regression.

Change-Id: I7c6859c8b68d9c6f7c0cbc8805c1f41dc5c1d5fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/169739


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