cmd/compile: always mark atColumn1 results as statements

In 31618, we end up comparing the is-stmt-ness of positions
to repurpose real instructions as inline marks. If the is-stmt-ness
doesn't match, we end up not being able to remove the inline mark.

Always use statement-full positions to do the matching, so we
always find a match if there is one.

Also always use positions that are statements for inline marks.

Fixes #31618

Change-Id: Idaf39bdb32fa45238d5cd52973cadf4504f947d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/173324
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/gc/inl.go

@@ -1055,7 +1055,7 @@ func mkinlcall(n, fn *Node, maxCost int32) *Node {
 	// to put a breakpoint. Not sure if that's really necessary or not
 	// (in which case it could go at the end of the function instead).
 	inlMark := nod(OINLMARK, nil, nil)
-	inlMark.Pos = n.Pos.WithDefaultStmt()
+	inlMark.Pos = n.Pos.WithIsStmt()
 	inlMark.Xoffset = int64(newIndex)
 	ninit.Append(inlMark)
 

src/cmd/compile/internal/gc/ssa.go

@@ -5369,6 +5369,7 @@ func genssa(f *ssa.Func, pp *Progs) {
 				// We found an instruction with the same source position as
 				// some of the inline marks.
 				// Use this instruction instead.
+				p.Pos = p.Pos.WithIsStmt() // promote position to a statement
 				pp.curfn.Func.lsym.Func.AddInlMark(p, inlMarks[m])
 				// Make the inline mark a real nop, so it doesn't generate any code.
 				m.As = obj.ANOP

src/cmd/internal/src/pos.go

@@ -445,5 +445,5 @@ func (x lico) lineNumberHTML() string {
 }
 
 func (x lico) atColumn1() lico {
-	return makeLico(x.Line(), 1) | (x & (isStmtMask | xlogueMask))
+	return makeLico(x.Line(), 1).withIsStmt()
 }

test/codegen/issue31618.go

@@ -0,0 +1,22 @@
+// asmcheck
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package codegen
+
+// Make sure we remove both inline marks in the following code.
+// Both +5 and +6 should map to real instructions, which can
+// be used as inline marks instead of explicit nops.
+func f(x int) int {
+	// amd64:-"XCHGL"
+	x = g(x) + 5
+	// amd64:-"XCHGL"
+	x = g(x) + 6
+	return x
+}
+
+func g(x int) int {
+	return x >> 3
+}