[MLIR][Shape] Merge `shape` to `std`/`scf` lowerings.

Merge the two lowering passes because they are not useful by themselves. The new
pass lowers to `std` and `scf` is considered an auxiliary dialect.

See also
https://llvm.discourse.group/t/conversions-with-multiple-target-dialects/1541/12

Differential Revision: https://reviews.llvm.org/D86779