[SelectionDAG] Add 2 tests for selection across basic blocks

Summary:
Add tests for selection across basic block boundary:
 * one test containing a buffer load, where part of the offset
   computation is placed in the predecessor of the load
 * similar test, but containing two buffer loads and shared
   computations

Please note that the behaviour being tested will be updated in
a subsequent commit.

This commit was extracted from https://reviews.llvm.org/D59535.

Reviewers: RKSimon

Reviewed By: RKSimon

Subscribers: jvesely, nhaehnle, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59690

llvm-svn: 357149