Fix undefined behaviour in PPCInstPrinter::printBranchOperand.

Fix the undefined behaviour introduced by my previous patch r353865 (left
shifting a potentially negative value), which was caught by the bots that run
UBSan.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353874 91177308-0d34-0410-b5e6-96231b3b80d8