[LSR] Generate cross iteration indexes

[LSR] Generate cross iteration indexes

Modify GenerateConstantOffsetsImpl to create offsets that can be used
by indexed addressing modes. If formulae can be generated which
result in the constant offset being the same size as the recurrence,
we can generate a pre-indexed access. This allows the pointer to be
updated via the single pre-indexed access so that (hopefully) no
add/subs are required to update it for the next iteration. For small
cores, this can significantly improve performance DSP-like loops.

Differential Revision: https://reviews.llvm.org/D55373

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353403 91177308-0d34-0410-b5e6-96231b3b80d8