[EarlyCSE] Simplify and invert ParseMemoryInst [NFCI]

Restructure ParseMemoryInst - which was introduced to abstract over target specific load and stores instructions - to just query the underlying instructions. In theory, this could be slightly slower than caching the results, but in practice, it's very unlikely to be measurable.

The simple query scheme makes it far easier to understand, and much easier to extend with new queries. Given I'm about to need to add new query types, doing the cleanup first seemed worthwhile.

Do we still believe the target specific intrinsic handling is worthwhile in EarlyCSE? It adds quite a bit of complexity and makes the code harder to read. Being able to delete the abstraction entirely would be wonderful.

llvm-svn: 254950