as vector shuffles did not work out well. Shuffles that produce double-wide
vectors accurately represent the operation but make it hard to do anything
with the results. I considered splitting them up into 2 shuffles, one to
write each register separately, but there doesn't seem to be a good way to
reunite them for codegen.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78437 91177308-0d34-0410-b5e6-96231b3b80d8