[clang] [CIR] Add if statement support (PR #134333)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

> @Andres-Salamanca thanks for working on this. We usually write tests that exercise any possible different paths codegen can take. For next rounds: it's fine if you add less things in one go and make it more incremental with the use of (a) errorNYI and (b) assert on missing features.

We should probably make it a standard practice when upstreaming new ops like this that if the change goes above 500 lines or so, we should leave the lowering to LLVM IR and related transformations for a follow-up PR.

https://github.com/llvm/llvm-project/pull/134333