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

[Bruno Cardoso Lopes via cfe-commits]

bcardosolopes 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.

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