Handle read-only scalars used in PHI-nodes correctly

This change addresses three issues:

  - Read only scalars that enter a PHI node through an edge that comes from
    outside the scop are not modeled any more, as such PHI nodes will always
    be initialized to this initial value right before the SCoP is entered.
  - For PHI nodes that depend on a scalar value that is defined outside the
    scop, but where the scalar values is passed through an edge that itself
    comes from a BB that is part of the region, we introduce in this basic
    block a read of the out-of-scop value to ensure it's value is available
    to write it into the PHI alloc location.
  - Read only uses of scalars by PHI nodes are ignored in the general read only
    handling code, as they are taken care of by the general PHI node modeling
    code.

llvm-svn: 248535