Add isBGQ method to PPCSubtarget

This function will be used in future commits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173729 91177308-0d34-0410-b5e6-96231b3b80d8
Unroll again after running BBVectorize

Because BBVectorize may significantly shorten a loop body, unroll
again after vectorization. This is especially important when using
runtime or partial unrolling.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173730 91177308-0d34-0410-b5e6-96231b3b80d8