Rollup merge of #70189 - RalfJung:is_signed, r=eddyb

Abi::is_signed: assert that we are a Scalar

A bit more sanity checking, suggested by @eddyb. This makes this method actually "safer" than `TyS::is_signed`, so I made sure Miri consistently uses the `Abi` version.

Though I am not sure if this would have caught the mistake where the layout of a zero-sized enum was asked for its sign.

r? @eddyb

diff --cc src/librustc_mir/interpret/operand.rs
Simple merge

diff --cc src/librustc_target/abi/mod.rs
Simple merge