[InstCombine][GC] Handle gc.relocations of vector type

We introduced gc.relocates of vector-of-pointer types a couple of weeks back.  Somehow, I missed updating the InstCombine rule to account for this.  If we hit this code path with a vector-of-pointers gc.relocate, we'd crash on a cast<PointerType>.

I also took the chance to do a bit of code style cleanup.

llvm-svn: 260279