Turn on earlycse by default. This seems to be a small performance

Turn on earlycse by default.  This seems to be a small performance
improvement in the generated code, and speeds up 'opt -std-compile-opts'
compile time on 176.gcc from 24.84s to 23.2s (about 7%).

This also resolves a specific code quality issue in rdar://7352081 which
was generating poor code for:

int t(int a, int b) {
  if (a & b & 1)
    return a & b;
  return 3;
}

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122740 91177308-0d34-0410-b5e6-96231b3b80d8