cmd/compile: adjust line numbering for type switch

Two changes, one to cause the back end to not number ITab operations
(these tend to disappear and are also followed by something more robust)
and to explicitly unmark (as statements) all but the first bit of
the code generated to implement a type switch.

Change-Id: I9f7bf7cbf7ccc5d7eda57f7fb080e600eb312eb0
Reviewed-on: https://go-review.googlesource.com/c/go/+/198739
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

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

@@ -513,6 +513,7 @@ func walkTypeSwitch(sw *Node) {
 	// Use a similar strategy for non-empty interfaces.
 	ifNil := nod(OIF, nil, nil)
 	ifNil.Left = nod(OEQ, itab, nodnil())
+	lineno = lineno.WithNotStmt() // disable statement marks after the first check.
 	ifNil.Left = typecheck(ifNil.Left, ctxExpr)
 	ifNil.Left = defaultlit(ifNil.Left, nil)
 	// ifNil.Nbody assigned at end.

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

@@ -15,7 +15,7 @@ func isPoorStatementOp(op Op) bool {
 	switch op {
 	// Note that Nilcheck often vanishes, but when it doesn't, you'd love to start the statement there
 	// so that a debugger-user sees the stop before the panic, and can examine the value.
-	case OpAddr, OpLocalAddr, OpOffPtr, OpStructSelect, OpPhi,
+	case OpAddr, OpLocalAddr, OpOffPtr, OpStructSelect, OpPhi, OpITab, OpIData,
 		OpIMake, OpStringMake, OpSliceMake, OpStructMake0, OpStructMake1, OpStructMake2, OpStructMake3, OpStructMake4,
 		OpConstBool, OpConst8, OpConst16, OpConst32, OpConst64, OpConst32F, OpConst64F:
 		return true