[mlir] Split alloc-like op LLVM lowerings into base and separate derived classes.

The previous code did the lowering to alloca, malloc, and aligned_malloc
in a single class with different code paths that are somewhat difficult to
follow.

This change moves the common code to a base class and has a separte
derived class per lowering target that contains the specifics.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D88696