[AArch64] Add getCFInstrCost, treat branches as free for throughput.

D79164/2596da31 changed getCFInstrCost to return 1 per default.
AArch64 did not have its own implementation, hence the throughput cost
of CFI instructions is overestimated. On most cores, most branches should
be predicated and essentially free throughput wise.

This restores a 9% performance regression on a SPEC2006 benchmark on
AArch64 with -O3 LTO & PGO.

This patch effectively restores pre 2596da31 behavior for AArch64
and undoes the AArch64 test changes of the patch.

Reviewers: samparker, dmgreen, anemet

Reviewed By: samparker

Differential Revision: https://reviews.llvm.org/D82755