Wrong local string varialbe allocation

[cpunion]

package main

func Foo(s string) int {
	return len(s)
}

func Test() {
	for i := 0; i < 1000000; i++ {
		_ = Foo("hello")
	}
}

func main() {
	Test()
}

Run with:

$ llgo build -o string . && ./string
# github.com/goplus/llgo/_demo/string
[1]    69802 segmentation fault  ./string

Generated code by llgen:

define void @main.Test() {
_llgo_0:
  br label %_llgo_3

_llgo_1:                                          ; preds = %_llgo_3
  %0 = alloca %"github.com/goplus/llgo/internal/runtime.String", align 8
  %1 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %0, i32 0, i32 0
  store ptr @0, ptr %1, align 8
  %2 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %0, i32 0, i32 1
  store i64 5, ptr %2, align 4
  %3 = load %"github.com/goplus/llgo/internal/runtime.String", ptr %0, align 8
  %4 = call i64 @main.Foo(%"github.com/goplus/llgo/internal/runtime.String" %3)
  %5 = add i64 %6, 1
  br label %_llgo_3

_llgo_2:                                          ; preds = %_llgo_3
  ret void

_llgo_3:                                          ; preds = %_llgo_1, %_llgo_0
  %6 = phi i64 [ 0, %_llgo_0 ], [ %5, %_llgo_1 ]
  %7 = icmp slt i64 %6, 1000000
  br i1 %7, label %_llgo_1, label %_llgo_2
}

Allocating memory by alloca on the stack in each iteration will exhaust the stack.