Fix swapped BasePtr and Offset in pre-inc memory addresses.

PPCTargetLowering::getPreIndexedAddressParts currently provides
the base part of a memory address in the offset result, and the
offset part in the base result.  That swap is then undone again
when an MI instruction is generated (in PPCDAGToDAGISel::Select
for loads, and using .md Pat patterns for stores).

This patch reverts this double swap, to make common code and
back-end be in sync as to which part of the address is base
and which is offset.

To avoid performance regressions in certain cases, target code
now checks whether the choice of base register would be rejected
for pre-inc accesses by common code, and attempts to swap base
and offset again in such cases.  (Overall, this means that now
pre-ice accesses are generated *more* frequently than before.)

llvm-svn: 177733