Prevent complex access ranges with low number of pieces.

  Previously we checked the number of pieces to decide whether or not a
  invariant load was to complex to be generated. However, there are
  cases when e.g., divisions cause the complexity to spike regardless of
  the number of pieces. To this end we now check the number of totally
  involved dimensions which will increase with the number of pieces but
  also the number of divisions.

llvm-svn: 269045