Auto merge of #137058 - scottmcm:trunc-unchecked, r=nikic

Emit `trunc nuw` for unchecked shifts and `to_immediate_scalar`

- For shifts this shrinks the IR by no longer needing an `assume` while still providing the UB information
- Having this on the `i8`→`i1` truncations will hopefully help with some places that have to load `i8`s or pass those in LLVM structs without range information

Author: bors