- Rename isLegalMemOpType to isSafeMemOpType. "Legal" is a very overloade term.

Also added more comments to explain why it is generally ok to return true.
- Rename getOptimalMemOpType argument IsZeroVal to ZeroOrLdSrc. It's meant to
be true for loaded source (memcpy) or zero constants (memset). The poor name
choice is probably some kind of legacy issue.

llvm-svn: 169954