Rollup merge of #138619 - yotamofek:pr/codegen_ssa/flatten-ifs, r=lcnr

Flatten `if`s in `rustc_codegen_ssa`

Best viewed [while ignoring whitespace](https://github.com/rust-lang/rust/pull/138619/files?diff=unified&w=1)

Author: matthiaskrgr