Align copied load/store instructions as the original.

  This also forbids the json importer to access other memory locations
  than the original instruction as we to reuse the alignment of the
  original load/store.

Differential Revision: http://reviews.llvm.org/D5560



git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@218883 91177308-0d34-0410-b5e6-96231b3b80d8