[mlir][Complex] Fix bug in `MergeComplexBitcast` (#74271)

When two `complex.bitcast` ops are folded and the resulting bitcast is a
non-complex -> non-complex bitcast, an `arith.bitcast` should be
generated. Otherwise, the generated `complex.bitcast` op is invalid.

Also remove a pattern that convertes non-complex -> non-complex
`complex.bitcast` ops to `arith.bitcast`. Such `complex.bitcast` ops are
invalid and should not appear in the input.

Note: This bug can only be triggered by running with `-debug` (which
will should intermediate IR that does not verify) or with
`MLIR_ENABLE_EXPENSIVE_PATTERN_API_CHECKS` (#74270).