Auto merge of #138391 - scottmcm:SSA-discriminants, r=WaffleLapkin

Don't `alloca` just to look at a discriminant

Today we're making LLVM do a bunch of extra work when you match on trivial stuff like `Option<bool>` or `ControlFlow<u8>`.

This PR changes that so that simple types like `Option<u32>` or `Result<(), Box<Error>>` can stay as `OperandValue::ScalarPair` and we can still read the discriminant from them, rather than needing to write them into memory to have a `PlaceValue` just to get the discriminant out.

Fixes #137503

Author: bors