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.

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