[PowerPC] Add branch flag on asm parser-only branch instructions

This flag was missing but it wasn't an issue as nothing depended on it
for these asm parser-only instructions. Now that LLDB support is slowly
landing, it is important to get this right.
Committing on behalf of Leonardo Bianconi.

Differential revision: https://reviews.llvm.org/D40846

llvm-svn: 320475