[EarlyCSE] IsSimple vs IsVolatile naming clarification (NFC)

[EarlyCSE] IsSimple vs IsVolatile naming clarification (NFC)

When the notion of target specific memory intrinsics was introduced to EarlyCSE, the commit confused the notions of volatile and simple memory access.  Since I'm about to start working on this area, cleanup the naming so that patches aren't horribly confusing.  Note that the actual implementation was always bailing if the load or store wasn't simple.

Reminder:
- "volatile" - C++ volatile, can't remove any memory operations, but in principal unordered
- "ordered" - imposes ordering constraints on other nearby memory operations
- "atomic" - can't be split or sheared.  In LLVM terms, all "ordered" operations are also atomic so the predicate "isAtomic" is often used.
- "simple" - a load which is none of the above.  These are normal loads and what most of the optimizer works with.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254805 91177308-0d34-0410-b5e6-96231b3b80d8