[PowerPC][NFC]Add testcase for STWU scheduling check

This patch add a STWU testcase for scheduling check.

Currently P7/P8 which use itineraries are missing IIC_LdStStoreUpd, 
We use CHECK-ITIN prefix to check P7/P8, then use default for P9 (and future).

We will fix the missing itineraries of IIC_LdStStoreUpd in following patch, 
and update this testcase to show the scheduling difference only there.

Differential Revision: https://reviews.llvm.org/D54699

llvm-svn: 347310