[PowerPC] eliminate unconditional branch to the next instruction

This patch makes analyzeBranch eliminate unconditional branch to the next instruction.
After basic blocks are re-organized by optimizers, such as machine block placement, a BB may end with an unconditional branch to the next (fallthrough) BB. This patch removes such redundant branch instruction.

Differential Revision: https://reviews.llvm.org/D37730

llvm-svn: 314297