[X86] Tag PSLLDQ/PSRLDQ as WriteShuffle scheduler classes instead of shifts.

Although they are encoded similar to bit shifts, the byte shifts behave like shuffles from a scheduling point of view.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331253 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed