[PowerPC] Ensure displacements for DQ-Form instructions are multiples of 16

[PowerPC] Ensure displacements for DQ-Form instructions are multiples of 16

As outlined in the PR, we didn't ensure that displacements for DQ-Form
instructions are multiples of 16. Since the instruction encoding encodes
a quad-word displacement, a sub-16 byte displacement is meaningless and
ends up being encoded incorrectly.

Fixes https://bugs.llvm.org/show_bug.cgi?id=33671.

Differential Revision: https://reviews.llvm.org/D35007

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307934 91177308-0d34-0410-b5e6-96231b3b80d8