Rollup merge of #130735 - compiler-errors:validate-unsize, r=spastorino,RalfJung

Simple validation for unsize coercion in MIR validation

This adds the most basic validity check to unsize coercions in MIR. The src and target of an unsize cast must *at least* implement `Src: CoerceUnsized<Target>` for this to be valid.

This doesn't the second, more subtle validity check that is taken of advantage in codegen [here](https://github.com/rust-lang/rust/blob/914193c8f40528fe82696e1054828de8c399882e/compiler/rustc_codegen_ssa/src/base.rs#L126), but I did leave a beefy FIXME for that explaining what it is.

As a consequence, this also fixes an ICE with GVN and invalid unsize coercions. This is somewhat coincidental, since MIR inlining will check that a body is valid before inlining it; so now that we determine it to be invalid, we don't inline it, and we don't encounter the GVN ICE. I'm not certain if the same GVN ICE is triggerable without the inliner, and perhaps instead with trivial where clauses or something.

cc `@RalfJung`

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