Fix ICE when transmute Assume field is invalid
This PR fixes an internal compiler error in `rustc_transmute` where initializing an `Assume` field (like `alignment`) with a non-scalar constant (like a struct) caused a panic.
The fix updates `from_const` to use `try_to_scalar()` instead of assuming the value is always a leaf. It now gracefully returns `None` for invalid types, allowing the compiler to report standard "missing field initialiser" errors instead of crashing.
Fixesrust-lang/rust#150506
58b1130a18