`& ref ident` patterns are buggy with DSTs

[Stebalien]

Aborts (no backtrace):

struct Test([i32]);
let t: &'static Test = unsafe { std::mem::transmute(&[0i32] as &[i32]) };
let & ref a = t;

With:

rustc: /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/llvm/include/llvm/Support/Casting.h:237: typename llvm::cast_retty<X, Y*>::ret_type llvm::cast(Y*) [with X = llvm::PointerType; Y = llvm::Type; typename llvm::cast_retty<X, Y*>::ret_type = llvm::PointerType*]: Assertion `isa<X>(Val) && "cast<Ty>() argument of incompatible type!"' failed.

Forked from: #30240.

[Stebalien]

According to @Aatch,

For the third error, the issue is due to an inconsistency between how the & pattern handles DSTs and how ref ident expects DST values to be given to it, so & ref ident has the outer pattern passing along a different value to the one the inner inner pattern expects, yay! I fixed one case in #30245, but not the one you're hitting here.