[mlir][bufferization] Simplify `getBufferType`

`getBufferType` computes the bufferized type of an SSA value without bufferizing any IR. This is useful for predicting the bufferized type of iter_args of a loop.

To avoid endless recursion (e.g., in the case of "scf.for", the type of the iter_arg depends on the type of init_arg and the type of the yielded value; the type of the yielded value depends on the type of the iter_arg again), `fixedTypes` was used to fall back to "fixed" type. A simpler way is to maintain an "invocation stack". `getBufferType` implementations can then inspect the invocation stack to detect repetitive computations (typically when computing the bufferized type of a block argument).

Also improve error messages in case of inconsistent memory spaces inside of a loop.

Differential Revision: https://reviews.llvm.org/D158060

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