Auto merge of #143860 - scottmcm:transmute-always-rvalue, r=WaffleLapkin

Let `codegen_transmute_operand` just handle everything

When combined with rust-lang/rust#143720, this means `rvalue_creates_operand` can just return `true` for *every* `Rvalue`.  (A future PR could consider removing it, though just letting it optimize out is fine for now.)

It's nicer anyway, IMHO, because it avoids needing the layout checks to be consistent in the two places, and thus is an overall reduction in code.  Plus it's a more helpful building block when used in other places this way.

(TBH, it probably would have been better to have it this way the whole time, but I clearly didn't understand `rvalue_creates_operand` when I originally wrote rust-lang/rust#109843.)

Author: bors