[GVN] Recommit the patch "Add phi-translate support in scalarpre".

[GVN] Recommit the patch "Add phi-translate support in scalarpre".

The recommit fixes three bugs: The first one is to use CurrentBlock instead of
PREInstr's Parent as param of performScalarPREInsertion because the Parent
of a clone instruction may be uninitialized. The second one is stop PRE when
CurrentBlock to its predecessor is a backedge and an operand of CurInst is
defined inside of CurrentBlock. The same value defined inside of loop in last
iteration can not be regarded as available. The third one is an out-of-bound
array access in a flipped if guard.

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306313 91177308-0d34-0410-b5e6-96231b3b80d8