[MLIR][Tosa] Turn reshape(const()) from canonicalization into fold; fix dynamic shape case

1) Turns the canonicalization into a fold, so it can cleanup IR within other passes.

2) When the output of the reshape is a dynamic shaped tensor,
we cannot apply the fold to the constant, because constants are required to have
static shape.

Differential Revision: https://reviews.llvm.org/D154615