Revert "Increase the size of this SmallVector in PeepholeOptimizer." (r215340)

That broke the build:

/data/buildslave/clang-amd64-freebsd/src-llvm/lib/CodeGen/PeepholeOptimizer.cpp:729:46: error: non-const lvalue reference to type 'SmallPtrSet<[...], 8>' cannot bind to a value of unrelated type 'SmallPtrSet<[...], 16>'
        Changed |= optimizeExtInstr(MI, MBB, LocalMIs);
                                             ^~~~~~~~
/data/buildslave/clang-amd64-freebsd/src-llvm/lib/CodeGen/PeepholeOptimizer.cpp:265:49: note: passing argument to parameter 'LocalMIs' here
                 SmallPtrSet<MachineInstr*, 8> &LocalMIs) {
                                                ^