Fix code generation of materialized conditions.

Move the logic for knowing if a condition needs to be materialized
in an optimization pass (so that the information does not change
as a side effect of another optimization).

Also clean-up arm and x86_64 codegen:
- arm: ldr and str are for power-users when a constant is
  in play. We should use LoadFromOffset and StoreToOffset.
- x86_64: fix misuses of movq instead of movl.

Change-Id: I01a03b91803624be2281a344a13ad5efbf4f3ef3