[NFC][PowerPC]Change ADDIStocHA to ADDIStocHA8 to follow 64-bit naming convention

Summary:

Since we are planning to add ADDIStocHA for 32bit in later patch, we decided
 to change 64bit one first to follow naming convention with 8 behind opcode.

Patch by: Xiangling_L

Differential Revision: https://reviews.llvm.org/D64814

llvm-svn: 366731