There are two motivations for this change:
1. It considerably simplifies adding support for the realloc operation to the
new buffer deallocation pass by lowering the realloc such that no
deallocation operation is inserted and the deallocation pass itself can
insert that dealloc
2. The lowering is expressed on a higher level and thus easier to understand,
and the lowerings of the memref operations it is composed of don't have to
be duplicated in the MemRefToLLVM lowering (also see discussion in
https://reviews.llvm.org/D133424)
Reviewed By: springerm
Differential Revision: https://reviews.llvm.org/D159430
