Allow transmute casts in pre-runtime-MIR

[oli-obk]

r? @scottmcm

cc @BoxyUwU

turns out in #138393 I erroneously used transmute casts in

rust/compiler/rustc_mir_build/src/builder/matches/test.rs

Line 209 in fd3da4b

Rvalue::Cast(CastKind::Transmute, Operand::Copy(place), base),

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

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @davidtwco, @vakaras

[scottmcm]

I think r=me for the code change here, since it looks like tests pass.

That said, I could have sworn there was more that would have been needed here. If this is in built mir now, are we sure that there's nothing in borrow-checking or move-checking or something that would need handling? (If not and I'm just worried for nothing that's fine too.)