[AArch64] Break the dependency between FP and SP when possible.

When the SP in not changed because of realignment/VLAs etc., we restore the SP
by using the previous value of SP and not the FP. Breaking the dependency will
help in cases when the epilog of a callee is close to the epilog of the caller;
for then "sub sp, fp, #" depends on the load restoring the FP in the epilog of
the callee.

http://reviews.llvm.org/D18060
Patch by Aditya Kumar and Evandro Menezes.

llvm-svn: 263458