Thumb2 Backend: Use sp-relative offsets for stack slot operations in Crankshaft

Thumb2 Backend: Use sp-relative offsets for stack slot operations in Crankshaft

This change converts loads and stores to stack slots and arguments
in Crankshaft code from fp-relative loads to sp-relative loads.
The distance between fp and sp is tracked in order to accomplish
this. The distance is the sum of the stack slots size, the pushed
arguments size, and a fixed frame size (8).
This allows more ldr/str instructions to use a 2-byte encoding
in Thumb2 mode.

This is the 11th patch for the Thumb2 Backend. 
Previous patch: https://codereview.chromium.org/24793002/

BUG=none
TEST=none