Clean up ArmMirToLir::LoadDispBody()/StoreDispBody().

Refactor the 64-bit load and store code to use a shared
helper function that can be used for any opcode with the
displacement limited to 8-bit value shifted by 2 (i.e. max
1020). Use that function also for 32-bit float load and
store as it is actually better than the old code for
offsets exceeding the 1020 byte limit.

Change-Id: I7dec38bae8cd9891420d2e92b1bac6138af5d64e