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, 12 insertions, 0 deletions

diff --git a/src/codegen/llvm.zig b/src/codegen/llvm.zig
index d6515f9ec6..5d753c41cb 100644
--- a/src/codegen/llvm.zig
+++ b/src/codegen/llvm.zig
@@ -396,6 +396,7 @@ pub const LLVMIRModule = struct {
                 .condbr => try self.genCondBr(inst.castTag(.condbr).?),
                 .intcast => try self.genIntCast(inst.castTag(.intcast).?),
                 .load => try self.genLoad(inst.castTag(.load).?),
+                .loop => try self.genLoop(inst.castTag(.loop).?),
                 .not => try self.genNot(inst.castTag(.not).?),
                 .ret => try self.genRet(inst.castTag(.ret).?),
                 .retvoid => self.genRetVoid(inst.castTag(.retvoid).?),
@@ -559,6 +560,17 @@ pub const LLVMIRModule = struct {
         return null;
     }
 
+    fn genLoop(self: *LLVMIRModule, inst: *Inst.Loop) !?*const llvm.Value {
+        const loop_block = self.context.appendBasicBlock(self.llvm_func, "Loop");
+        _ = self.builder.buildBr(loop_block);
+
+        self.builder.positionBuilderAtEnd(loop_block);
+        try self.genBody(inst.body);
+
+        _ = self.builder.buildBr(loop_block);
+        return null;
+    }
+
     fn genNot(self: *LLVMIRModule, inst: *Inst.UnOp) !?*const llvm.Value {
         return self.builder.buildNot(try self.resolveInst(inst.operand), "");
     }