Use Thumb sp-relative store.
Use Thumb sp-relative store.

When storing a register to stack, we used to always generate Thumb2 store.
This CL uses a kThumbStrSpRel instruction when generating sp-relative stores
with a Thumb source register.

Change-Id: If729b5428c41350f8dc253e71ce8b63acbbc2acc

treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst