runtime: remove scase.releasetime field

This is the gofrontend version of https://golang.org/cl/245122.
Original CL description:

    selectgo will report at most one block event, so there's no need to
    keep a releasetime for every select case. It suffices to simply track
    the releasetime of the case responsible for the wakeup.

    Updates golang/go#40410.

This is being brought over to gofrontend as a step toward upgrading to
Go1.16beta1.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/279732

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-f9e44c6e510df9f047dbe28eb5fa19642e350f8f
+c8456995b0118a92820c6c1d8f996d4b1adf55c2
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/types.cc

@@ -8908,11 +8908,10 @@ Channel_type::select_case_type()
       Type* uint16_type = Type::lookup_integer_type("uint16");
       Type* int64_type = Type::lookup_integer_type("int64");
       scase_type =
-	Type::make_builtin_struct_type(4,
+	Type::make_builtin_struct_type(3,
 				       "c", unsafe_pointer_type,
 				       "elem", unsafe_pointer_type,
-				       "kind", uint16_type,
-				       "releasetime", int64_type);
+				       "kind", uint16_type);
       scase_type->set_is_struct_incomparable();
     }
   return scase_type;

libgo/go/runtime/select.go

@@ -32,10 +32,9 @@ const (
 // Known to compiler.
 // Changes here must also be made in src/cmd/internal/gc/select.go's scasetype.
 type scase struct {
-	c           *hchan         // chan
-	elem        unsafe.Pointer // data element
-	kind        uint16
-	releasetime int64
+	c    *hchan         // chan
+	elem unsafe.Pointer // data element
+	kind uint16
 }
 
 func sellock(scases []scase, lockorder []uint16) {
@@ -151,9 +150,6 @@ func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
 	var t0 int64
 	if blockprofilerate > 0 {
 		t0 = cputicks()
-		for i := 0; i < ncases; i++ {
-			scases[i].releasetime = -1
-		}
 	}
 
 	// The compiler rewrites selects that statically have
@@ -241,6 +237,7 @@ loop:
 	var dfl *scase
 	var casi int
 	var cas *scase
+	var caseReleaseTime int64 = -1
 	var recvOK bool
 	for i := 0; i < ncases; i++ {
 		casi = int(pollorder[i])
@@ -361,13 +358,13 @@ loop:
 		if k.kind == caseNil {
 			continue
 		}
-		if sglist.releasetime > 0 {
-			k.releasetime = sglist.releasetime
-		}
 		if sg == sglist {
 			// sg has already been dequeued by the G that woke us up.
 			casi = int(casei)
 			cas = k
+			if sglist.releasetime > 0 {
+				caseReleaseTime = sglist.releasetime
+			}
 		} else {
 			c = k.c
 			if k.kind == caseSend {
@@ -465,8 +462,8 @@ send:
 	goto retc
 
 retc:
-	if cas.releasetime > 0 {
-		blockevent(cas.releasetime-t0, 1)
+	if caseReleaseTime > 0 {
+		blockevent(caseReleaseTime-t0, 1)
 	}
 
 	// Check preemption, since unlike gc we don't check on every call.