Allow multiple reductions per statement

  Iterate over all store memory accesses and check for valid binary reduction
  candidate loads by following the operands of the stored value.  For each
  candidate pair we check if they have the same base address and there are no
  other accesses which may overlap with them. This ensures that no intermediate
  value can escape into other memory locations or is overwritten at some point.

  + 17 test cases for reduction detection and reduction dependency modeling



git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@211957 91177308-0d34-0410-b5e6-96231b3b80d8