[NNC] Implement Cond in LLVM codegen (#47256)
Summary:
Generate LLVM IR for statements such as
```
if (...) {
....
} else {
....
}
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47256
Test Plan: added unit tests to test_llvm.cpp
Reviewed By: nickgg
Differential Revision: D24699080
Pulled By: cheng-chang
fbshipit-source-id: 83b0cebcd242828263eb6052483f0924b5f091ce
