[AMDGPU] Fix dwordx3/southern-islands failures.

This commit fixes the dwordx3/southern-islands failures that were found
in bugzilla https://bugs.llvm.org/show_bug.cgi?id=40129, by not
generating the dwordx3 variants of load/store instructions that were
added to the ISA after southern islands.

Differential Revision: https://reviews.llvm.org/D56434

llvm-svn: 350838