Clean up the atomic opcodes in SelectionDAG.

Clean up the atomic opcodes in SelectionDAG.

This removes all the _8, _16, _32, and _64 opcodes and replaces each
group with an unsuffixed opcode. The MemoryVT field of the AtomicSDNode
is now used to carry the size information. In tablegen, the size-specific
opcodes are replaced by size-independent opcodes that utilize the
ability to compose them with predicates.

This shrinks the per-opcode tables and makes the code that handles
atomics much more concise.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61389 91177308-0d34-0410-b5e6-96231b3b80d8