Infer instruction properties from single-instruction patterns.

Previously, instructions without a primary patterns wouldn't get their
properties inferred. Now, we use all single-instruction patterns for
inference, including 'def : Pat<>' instances.

This causes a lot of instruction flags to change.

- Many instructions no longer have the UnmodeledSideEffects flag because
  their flags are now inferred from a pattern.

- Instructions with intrinsics will get a mayStore flag if they already
  have UnmodeledSideEffects and a mayLoad flag if they already have
  mayStore. This is because intrinsics properties are linear.

- Instructions with atomic_load patterns get a mayStore flag because
  atomic loads can't be reordered. The correct workaround is to create
  pseudo-instructions instead of using normal loads. PR13693.

llvm-svn: 162614