Bug fix on function epilog optimization (ARM backend)

To save a 'add sp,#val' instruction by adding registers to the final pop instruction,
the first register transferred by this pop instruction need to be found.
If the function to be optimized has a non-void return value, the operand list contains
r0 (implicit) which prevents the optimization to take place.
Therefore implicit register references should be skipped in the search loop,
because this registers are never popped from the stack.

Patch by Rainer Herbertz (rOptimizer)!

Differential revision: https://reviews.llvm.org/D66730

llvm-svn: 370728