This CL removes two warts from the ARM32 backend:
1) during argument lowering, if a stack parameter is assigned a
register, the backend creates a new Variable that references the stack
location with the incoming argument, and _mov() it to the parameter.

2) During stack slot legalization, all _mov(Mem(), Reg) are converted to
stores; and all _mov(Reg, Mem()) are converted to loads.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4076
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/1457683004 .