[SimplifyCFG] try harder to forward switch condition to phi (PR34471)

The missed canonicalization/optimization in the motivating test from PR34471 leads to very different codegen:

  int switcher(int x) {
      switch(x) {
      case 17: return 17;
      case 19: return 19;
      case 42: return 42;
      default: break;
      }
      return 0;
    }

  int comparator(int x) {
    if (x == 17) return 17;
    if (x == 19) return 19;
    if (x == 42) return 42;
    return 0;
  }

For the first example, we use a bit-test optimization to avoid a series of compare-and-branch:
https://godbolt.org/g/BivDsw

Differential Revision: https://reviews.llvm.org/D39011

llvm-svn: 316293