Avoid high-latency false CPSR dependencies even for tMOVSi.

The Thumb2SizeReduction pass avoids false CPSR dependencies, except it
still aggressively creates tMOVi8 instructions because they are so
common.

Avoid creating false CPSR dependencies even for tMOVi8 instructions when
the the CPSR flags are known to have high latency. This allows integer
computation to overlap floating point computations.

Also process blocks in a reverse post-order and propagate high-latency
flags to successors.

<rdar://problem/13468102>