Implement double and float support for arm in register allocator.

The basic approach is:
- An instruction that needs two registers gets two intervals.
- When allocating the low part, we also allocate the high part.
- When splitting a low (or high) interval, we also split the high
  (or low) equivalent.
- Allocation follows the (S/D register) requirement that low
  registers are always even and the high equivalent is low + 1.

Change-Id: I06a5148e05a2ffc7e7555d08e871ed007b4c2797