PPC/CodeGen: fix lwa instruction generation

Authored by trommler on Oct 1 2016, 4:56 PM.

Description

PPC/CodeGen: fix lwa instruction generation

Opcode lwa is a 64-bit opcode and allows a DS-form only. This patch
generates lwa opcodes only when the offset is a multiple of 4.

Fixes Trac #12621

Test Plan: validate

Reviewers: erikd, hvr, simonmar, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2547

GHC Trac Issues: Trac #12621