Add PPC64 mulli pattern

The PPC backend had been missing a pattern to generate mulli for 64-bit
multiples. We had been generating it only for 32-bit multiplies. Unfortunately,
generating li + mulld unnecessarily increases register pressure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187807 91177308-0d34-0410-b5e6-96231b3b80d8