[Simplify] Implement write accesses coalescing.

Write coalescing combines write accesses that

- Write the same llvm::Value.
- Write to the same array.
- Unless they do not write anything in a statement instance (partial
  writes), write to the same element.
- There is no other access between them that accesses the same element.

This is particularly useful after DeLICM, which leaves partial writes to
disjoint domains.

Differential Revision: https://reviews.llvm.org/D36010

llvm-svn: 309489