cmd/compile: don't treat an InlMark as a read during deadstore

An InlMark "read" can't make an otherwise dead store live. Without this
CL, we sometimes zero an object twice in succession because we think
there is a reader in between.

Kind of challenging to make a test for this. The second zeroing has the
same instruction on the same line number, so codegen tests can't see it.

Fixes #67957

Change-Id: I7fb97ebff50d8eb6246fc4802d1136b7cc76c45f
Reviewed-on: https://go-review.googlesource.com/c/go/+/592615
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>

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

@@ -59,6 +59,10 @@ func dse(f *Func) {
 						continue
 					}
 				}
+				if v.Op == OpInlMark {
+					// Not really a use of the memory. See #67957.
+					continue
+				}
 				for _, a := range v.Args {
 					if a.Block == b && a.Type.IsMemory() {
 						loadUse.add(a.ID)

src/runtime/symtabinl_test.go

@@ -107,16 +107,17 @@ func lineNumber() int {
 // Below here is the test data for XTestInlineUnwinder
 
 var tiuStart = lineNumber() // +0
-var tiu1, tiu2, tiu3 int    // +1
+var tiu2, tiu3 int          // +1
-func tiuInlined1() { // +2
+func tiuInlined1(i int) { // +2
-	tiu1++ // +3
+	tiu1[i]++ // +3
 } // +4
 func tiuInlined2() { // +5
-	tiuInlined1() // +6
+	tiuInlined1(1) // +6
-	tiu2++        // +7
+	tiu2++         // +7
 } // +8
 func tiuTest() { // +9
-	tiuInlined1() // +10
+	tiuInlined1(0) // +10
-	tiuInlined2() // +11
+	tiuInlined2()  // +11
-	tiu3++        // +12
+	tiu3++         // +12
-} // +13
+}               // +13
+var tiu1 [2]int // +14