PPC/CodeGen: fix lwa instruction generation
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 #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: #12621

(cherry picked from commit ce3370e06165690e79a8eb22e5229b515157e00f)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc