[X86] Add WriteRotate schedule class, splitting off from WriteShift.

[X86] Add WriteRotate schedule class, splitting off from WriteShift.

NFCI for now, but it should make it easier to remove a lot of unnecessary overrides in a future commit.

Now that funnel shift intrinsics are coming online we need to get this cleaned up to make vectorization costs from scalar rotate patterns more straightforward.

llvm-svn: 342837