[llvm] [EarlyCSE] Add support for writeonly call CSE (PR #145474)

[Nikita Popov via llvm-commits]

nikic wrote:

This is a revival of https://reviews.llvm.org/D116609 by @d0k. This patch differs in a few ways:

 * Don't reset LastStore. This needs to be done for instructions that load memory, not for those that store memory.
 * Increment the memory generation in a way that makes trivial cases work without MemorySSA.
 * Don't handle storing meminsts, because the specialized handling for them is more precise.

https://github.com/llvm/llvm-project/pull/145474