[PeepholeOptimizer] Enable the advanced copy optimization by default.

The advanced copy optimization does not yield any difference on the whole llvm
test-suite + SPECs, either in compile time or runtime (binaries are identical),
but has a big potential when data go back and forth between register files as
demonstrated with test/CodeGen/ARM/adv-copy-opt.ll.

Note: This was measured for both Os and O3 for armv7s, arm64, and x86_64.

<rdar://problem/12702965>