[X86][Nearly NFC] Split SHLD/SHRD into their own WriteShiftDouble class

Summary:
{F6603964}
While there is still some discrepancies within that new group,
it is clearly separate from the other shifts.
And Agner's tables agree, these double shifts are clearly
different from the normal shifts/rotates.

I'm guessing `FeatureSlowSHLD` is related.

Indeed, a basic sched pair is *not* the /best/ match.
But keeping it in the WriteShift is /clearly/ not ideal either.
This can and likely will be fine-tuned later.

This is purely mechanical change, it does not change any numbers,
as the [lack of the change of] mca tests show.

Reviewers: craig.topper, RKSimon, andreadb

Reviewed By: craig.topper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D49015

llvm-svn: 336515