stage2: add support for loops in LLVM backend

A simple `while(true) {}` loop generates the following LLVMIR:
```
define i32 @main() {
Entry:
  br label %Loop

Loop:                                 ; preds = %Loop, %Entry
  br label %Loop
}
```

Also implement TZIR printing for loops and add a corresponding test.

1 files changed, 21 insertions, 0 deletions

diff --git a/test/stage2/llvm.zig b/test/stage2/llvm.zig
index eecea3d1f0..69622714a7 100644
--- a/test/stage2/llvm.zig
+++ b/test/stage2/llvm.zig
@@ -111,4 +111,25 @@ pub fn addCases(ctx: *TestContext) !void {
             \\}
         , "");
     }
+
+    {
+        var case = ctx.exeUsingLlvmBackend("while loops", linux_x64);
+
+        case.addCompareOutput(
+            \\fn assert(ok: bool) void {
+            \\    if (!ok) unreachable;
+            \\}
+            \\
+            \\export fn main() c_int {
+            \\    var sum: u32 = 0;
+            \\    var i: u32 = 0;
+            \\    while (i < 5) : (i += 1) {
+            \\        sum += i;
+            \\    }
+            \\    assert(sum == 10);
+            \\    assert(i == 5);
+            \\    return 0;
+            \\}
+        , "");
+    }
 }