[mlir][Linalg] Disable init_tensor elimination by default

init_tensor elimination is arguably a pre-optimization that should be separated from comprehensive bufferization.
In any case it is still experimental and easily results in wrong IR with violated SSA def-use orderings.
Isolate the optimization behind a flag, separate the test cases and add a test case that would results in wrong IR.

Differential Revision: https://reviews.llvm.org/D116936