[PowerPC][MC] Support expressions in getMemRIX16Encoding.

Loosens an assert in getMemRIX16Encoding that restricts DQ-form instructions to
using an immediate, so that we can assemble instructions like lxv/stxv where the
offset is an expression.

Differential Revision: https://reviews.llvm.org/D51122

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340761 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed