[DAGCombine] Move AND nodes to multiple load leaves

Search from AND nodes to find whether they can be propagated back to
loads, so that the AND and load can be combined into a narrow load.
We search through OR, XOR and other AND nodes and all bar one of the
leaves are required to be loads or constants. The exception node then
needs to be masked off meaning that the 'and' isn't removed, but the
loads(s) are narrowed still.

Differential Revision: https://reviews.llvm.org/D39604

llvm-svn: 319773