[x86] Fold together the two shift lowering strategies. They were doing

quite literally the same work, we just need to special case the >64-bit
element shift code emission to emit the byte shift instructions and
offsets. This also makes reasoning about each of the vector lowering
strategies easier as we don't have to remember to use both forms.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229662 91177308-0d34-0410-b5e6-96231b3b80d8