Allow `Rvalue::Repeat` to return true in `rvalue_creates_operand` too
The conversation in https://github.com/rust-lang/rust/pull/143502#discussion_r2189410911 made be realize how easy this is to handle, since the only possibilty is ZSTs -- everything else ends up with the destination being `LocalKind::Memory` and thus doesn't call `codegen_rvalue_operand` at all.
This gets us perilously close to a world where `rvalue_creates_operand` only ever returns true. (See rust-lang/rust#143860 for more.)
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