[Pipeliner] Enable more base+offset dependence changes in pipeliner

The pipeliner changes dependences between base+offset instructions
(loads and stores) so that the instructions have more flexibility
to be scheduled with respect to each other. This occurs when the
pipeliner is able to compute that the instructions will not alias
if their order is changed. The prevous code enforced the alias
property by checking if the base register is the same, and that the
offset values are either both positive or negative.

This patch improves the alias check by using the API
areMemAccessesTriviallyDisjoint instead. This enables more cases,
especially if the offset is a negative value. The pipeliner uses
the function by creating a new instruction with the offset used
in the next iteration.

Patch by Brendon Cahoon.

llvm-svn: 328538