[SelectionDAG] Teach SelectionDAGBuilder's getUniformBase for gather/scatter...

[SelectionDAG] Teach SelectionDAGBuilder's getUniformBase for gather/scatter handling to accept GEPs with more than 2 operands if the middle operands are all 0s

Currently we can only get a uniform base from a simple GEP with 2 operands. This causes us to miss address folding opportunities for simple global array accesses as the test case shows.

This patch adds support for larger GEPs if the other indices are 0 since those don't require any additional computations to be inserted.

We may also want to handle constant splats of zero here, but I'm leaving that for future work when I have a real world example.

Differential Revision: https://reviews.llvm.org/D39911

llvm-svn: 317947