compiler: do not emit nil checks for loading closure variables

Closure variables are allocated in a parent function and are thus never
nil. Don't do a nil check before reading or modifying the value.

This commit results in a slight reduction in code size in some test
cases: calls.go, channel.go, goroutines.go, json.go, sort.go -
presumably wherever closures are used.

compiler/asserts.go

@@ -162,6 +162,10 @@ func (b *builder) createNilCheck(inst ssa.Value, ptr llvm.Value, blockPrefix str
 	case *ssa.Alloc:
 		// An alloc is never nil.
 		return
+	case *ssa.FreeVar:
+		// A free variable is allocated in a parent function and is thus never
+		// nil.
+		return
 	case *ssa.IndexAddr:
 		// This pointer is the result of an index operation into a slice or
 		// array. Such slices/arrays are already bounds checked so the pointer

compiler/compiler.go

@@ -994,6 +994,8 @@ func getPos(val posser) token.Pos {
 	switch val := val.(type) {
 	case *ssa.MakeInterface:
 		return getPos(val.X)
+	case *ssa.MakeClosure:
+		return val.Fn.(*ssa.Function).Pos()
 	case *ssa.Return:
 		syntax := val.Parent().Syntax()
 		if syntax != nil {

compiler/testdata/goroutine-cortex-m-qemu.ll

@@ -86,14 +86,6 @@ entry:
 
 define hidden void @"main.closureFunctionGoroutine$1"(i32 %x, i8* %context, i8* %parentHandle) unnamed_addr {
 entry:
-  %0 = icmp eq i8* %context, null
-  br i1 %0, label %store.throw, label %store.next
-
-store.throw:                                      ; preds = %entry
-  call void @runtime.nilPanic(i8* undef, i8* null)
-  unreachable
-
-store.next:                                       ; preds = %entry
   %unpack.ptr = bitcast i8* %context to i32*
   store i32 7, i32* %unpack.ptr, align 4
   ret void
@@ -112,8 +104,6 @@ entry:
 
 declare void @runtime.printint32(i32, i8*, i8*)
 
-declare void @runtime.nilPanic(i8*, i8*)
-
 define hidden void @main.funcGoroutine(i8* %fn.context, void (i32, i8*, i8*)* %fn.funcptr, i8* %context, i8* %parentHandle) unnamed_addr {
 entry:
   %0 = call i8* @runtime.alloc(i32 12, i8* undef, i8* null)

compiler/testdata/goroutine-wasm.ll

@@ -62,14 +62,6 @@ entry:
 
 define hidden void @"main.closureFunctionGoroutine$1"(i32 %x, i8* %context, i8* %parentHandle) unnamed_addr {
 entry:
-  %0 = icmp eq i8* %context, null
-  br i1 %0, label %store.throw, label %store.next
-
-store.throw:                                      ; preds = %entry
-  call void @runtime.nilPanic(i8* undef, i8* null)
-  unreachable
-
-store.next:                                       ; preds = %entry
   %unpack.ptr = bitcast i8* %context to i32*
   store i32 7, i32* %unpack.ptr, align 4
   ret void
@@ -77,8 +69,6 @@ store.next:                                       ; preds = %entry
 
 declare void @runtime.printint32(i32, i8*, i8*)
 
-declare void @runtime.nilPanic(i8*, i8*)
-
 define hidden void @main.funcGoroutine(i8* %fn.context, i32 %fn.funcptr, i8* %context, i8* %parentHandle) unnamed_addr {
 entry:
   %0 = call i32 @runtime.getFuncPtr(i8* %fn.context, i32 %fn.funcptr, i8* nonnull @"reflect/types.funcid:func:{basic:int}{}", i8* undef, i8* null)

transform/testdata/allocs2.go

@@ -33,7 +33,7 @@ func main() {
 	c1 := getComplex128() // OUT: object allocated on the heap: escapes at line 34
 	useInterface(c1)
 
-	n3 := 5 // OUT: object allocated on the heap: escapes at line 39
+	n3 := 5
 	func() int {
 		return n3
 	}()
@@ -42,6 +42,13 @@ func main() {
 
 	s8 := []int{3, 5, 8} // OUT: object allocated on the heap: escapes at line 44
 	callVariadic(s8...)
+
+	n4 := 3 // OUT: object allocated on the heap: escapes at line 48
+	n5 := 7 // OUT: object allocated on the heap: escapes at line 48
+	func() {
+		n4 = n5
+	}()
+	println(n4, n5)
 }
 
 func derefInt(x *int) int {