cmd/compile: repair ssa/debug_test regression
cmd/compile: repair ssa/debug_test regression

Code introduced in

[2034fbab5b1d11bc59cb476bc3f49ee1b344839d]
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 <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

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