[MLIR] Simplify key construction/hashing in StorageUniquer

`getKey` and `getHash` use mutually exclusive overloads based on existence of
methods to determine how to compute get the key or hash, respectively.  This is
a bit verbose with `std::enable_if_t`.  Simplify it a bit by using
`if constexpr` directly.  As an added bonus, this is slightly quicker to
compile.

Differential Revision: https://reviews.llvm.org/D139245