stage2: add initial impl of control flow in LLVM backend

The following TZIR instrutions have been implemented in the backend:
- all cmp operators (lt, lte, gt, gte, eq, neq)
- block
- br
- condbr

The following LLVMIR is generated for a simple assert function:
```
define void @assert(i1 %0) {
Entry:
  %1 = alloca i1, align 1
  store i1 %0, i1* %1, align 1
  %2 = load i1, i1* %1, align 1
  %3 = xor i1 %2, true
  br i1 %3, label %Then, label %Else

Then:                                             ; preds = %Entry
  call void @llvm.debugtrap()
  unreachable

Else:                                             ; preds = %Entry
  br label %Block

Block:                                            ; preds = %Else
  ret void
}
```

See tests for more examples.

1 files changed, 71 insertions, 0 deletions

diff --git a/test/stage2/llvm.zig b/test/stage2/llvm.zig
index ebb5fc390f..eecea3d1f0 100644
--- a/test/stage2/llvm.zig
+++ b/test/stage2/llvm.zig
@@ -40,4 +40,75 @@ pub fn addCases(ctx: *TestContext) !void {
             \\}
         , "hello world!" ++ std.cstr.line_sep);
     }
+
+    {
+        var case = ctx.exeUsingLlvmBackend("simple if statement", linux_x64);
+
+        case.addCompareOutput(
+            \\fn add(a: i32, b: i32) i32 {
+            \\    return a + b;
+            \\}
+            \\
+            \\fn assert(ok: bool) void {
+            \\    if (!ok) unreachable;
+            \\}
+            \\
+            \\export fn main() c_int {
+            \\    assert(add(1,2) == 3);
+            \\    return 0;
+            \\}
+        , "");
+    }
+
+    {
+        var case = ctx.exeUsingLlvmBackend("blocks", linux_x64);
+
+        case.addCompareOutput(
+            \\fn assert(ok: bool) void {
+            \\    if (!ok) unreachable;
+            \\}
+            \\
+            \\fn foo(ok: bool) i32 {
+            \\    const val: i32 = blk: {
+            \\        var x: i32 = 1;
+            \\        if (!ok) break :blk x + 9;
+            \\        break :blk x + 19;
+            \\    };
+            \\    return val + 10;
+            \\}
+            \\
+            \\export fn main() c_int {
+            \\    assert(foo(false) == 20);
+            \\    assert(foo(true) == 30);
+            \\    return 0;
+            \\}
+        , "");
+    }
+
+    {
+        var case = ctx.exeUsingLlvmBackend("nested blocks", linux_x64);
+
+        case.addCompareOutput(
+            \\fn assert(ok: bool) void {
+            \\    if (!ok) unreachable;
+            \\}
+            \\
+            \\fn foo(ok: bool) i32 {
+            \\    var val: i32 = blk: {
+            \\        const val2: i32 = another: {
+            \\            if (!ok) break :blk 10;
+            \\            break :another 10;
+            \\        };
+            \\        break :blk val2 + 10;
+            \\    };
+            \\    return val;
+            \\}
+            \\
+            \\export fn main() c_int {
+            \\    assert(foo(false) == 10);
+            \\    assert(foo(true) == 20);
+            \\    return 0;
+            \\}
+        , "");
+    }
 }