[GVN] Add phi-translate support in scalarpre.

Right now scalarpre doesn't have phi-translate support, so it will miss some
simple pre opportunities. Like the following testcase, current scalarpre cannot
recognize the last "a * b" is fully redundent because a and b used by the last
"a * b" expr are both defined by phis.

  long a[100], b[100], g1, g2, g3;
  __attribute__((pure)) long goo();

  void foo(long a, long b, long c, long d) {
    g1 = a * b;
    if (__builtin_expect(g2 > 3, 0)) {
      a = c;
      b = d;
      g2 = a * b;
    }
    g3 = a * b;      // fully redundant.
  }

The patch adds phi-translate support in scalarpre. This is only a temporary
solution before the newpre based on newgvn is available.

Differential Revision: https://reviews.llvm.org/D32252

llvm-svn: 303923