[Compiler] Exit-path bug fix
[Compiler] Exit-path bug fix

Surprised that we haven't run into this before.  The exit code for
a method involves stripping the frame.  For frame sizes < 1024 this
can be accomplished with an add sp,#imm.  For larger frames we need
to materialize the frame size in a temp register and then do a
register to register add.

The exit path code was failing to ensure that r0/r1 were not used
as temps in the large frame case - otherwise the return value could
be clobbered.

Change-Id: If2618e9522fb10597f0f8c8e8afffd20d73b5d37

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