transform: improve GC stack slot pass to work around a bug

Bug 1790 ("musttail call must precede a ret with an optional bitcast")
is caused by the GC stack slot pass inserting a store instruction
between a musttail call and a return instruction. This is not allowed in
LLVM IR.

One solution would be to remove the musttail. That would probably work,
but 1) the go-llvm API doesn't support this and 2) this might have
unforeseen consequences. What I've done in this commit is to move the
store instruction to a position earlier in the basic block, just after
the last access to the GC stack slot alloca.

Thanks to @fgsch for a very small repro, which I've used as a regression
test.

testdata/goroutines.go

@@ -76,6 +76,8 @@ func main() {
 	testGoOnBuiltins()
 
 	testCond()
+
+	testIssue1790()
 }
 
 func acquire(m *sync.Mutex) {
@@ -198,3 +200,13 @@ func testCond() {
 		panic("missing queued notification")
 	}
 }
+
+var once sync.Once
+
+// This tests a fix for issue 1790:
+// https://github.com/tinygo-org/tinygo/issues/1790
+func testIssue1790() *int {
+	once.Do(func() {})
+	i := 0
+	return &i
+}