[Pipeliner] Fix offset value for instrs dependent on post-inc load/stores

The software pipeliner and the packetizer try to break dependence
between the post-increment instruction and the dependent memory
instructions by changing the base register and the offset value.
However, in some cases, the existing logic didn't work properly
and created incorrect offset value.

Patch by Jyotsna Verma.

llvm-svn: 315468