[CGP] Ensure splitgep gives deterministic output

The output of splitLargeGEPOffsets does not appear to be deterministic because
of the way that we iterate over a DenseMap. I've changed it to a MapVector for
consistent output.

The test here isn't particularly great, only showing a consmetic difference in
output. The original reproducer is much larger but show a diffierence in
instruction ordering, leading to different codegen.

Differential Revision: https://reviews.llvm.org/D51851

llvm-svn: 342043