Avoids ICEs such as the following:
error: internal compiler error: src\librustc_metadata\cstore_impl.rs:131:
get_optimized_mir: missing MIR for `DefId(8/0:40 ~
compiler_builtins[9532]::int[0]::addsub[0]::rust_i128_addo[0])`
* The overflow-checking shift items need to take a full 128-bit type, since they need to be able to detect idiocy like `1i128 << (1u128 << 127)`
* The unchecked ones just take u32, like the `*_sh?` methods in core
* Because shift-by-anything is allowed, cast into a new local for every shift
