ScopInfo: Split start value from SCEVAddRecExpr to enable parameter sharing.

SCoP invariant parameters with the different start value would deter parameter
sharing. For example, when compiling the following C code:

  void foo(float *input) {
    for (long j = 0; j < 8; j++) {
      // SCoP begin
      for (long i = 0; i < 8; i++) {
        float x = input[j * 64 + i + 1];
        input[j * 64 + i] = x * x;
      }
    }
  }

Polly would creat two parameters for these memory accesses:

    p_0: {0,+,256}
    p_2: {4,+,256}
    [j * 64 + i + 1] => MemRef_input[o0] : 4o0 = p_1 + 4i0
    [j * 64 + i]     => MemRef_input[o0] : 4o0 = p_0 + 4i0

These parameters only differ from start value. To enable parameter sharing,
we split the start value from SCEVAddRecExpr, so they would share a single
parameter that always has zero start value:

    p0: {0,+,256}<%for.cond1.preheader>
    [j * 64 + i + 1] => MemRef_input[o0] : 4o0 = 4 + p_1 + 4i0
    [j * 64 + i]     => MemRef_input[o0] : 4o0 = p_0 + 4i0

Such translation can make the polly-dependence much faster.

Contributed-by: Star Tan <tanmx_star@yeah.net>