Rollup merge of #142124 - oli-obk:transmute-cast, r=scottmcm

Allow transmute casts in pre-runtime-MIR

r? ``@scottmcm``

cc ``@BoxyUwU``

turns out in rust-lang/rust#138393 I erroneously used transmute casts in https://github.com/rust-lang/rust/blob/fd3da4bebdff63b7529483ff7025986ef16bf463/compiler/rustc_mir_build/src/builder/matches/test.rs#L209

I don't think they have any issues using them before runtime, we just checked for them because we didn't have code exercising those code paths

Author: fmease