PPC: Add some missing V_SET0 patterns

We had patterns to match v4i32 immAllZerosV -> V_SET0, but not patterns for
v8i16 (which occurs in the test case) or v16i8. The same was true for
V_SETALLONES (so I added the associated patterns for those as well).

Another bug found by llvm-stress.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186108 91177308-0d34-0410-b5e6-96231b3b80d8