[X86] Move X86ShuffleDecode.cpp/h into MCTargetDesc and remove X86Utils library. NFC

The shuffle decoding is used by X86ISelLowering and
MCTargetDesc/X86InstComments. The latter used to be in a
separate InstPrinter library. The Utils library existed to allow
InstPrinter and CodeGen to share the shuffle decoding. Since
X86InstComments now lives in the MCTargetDesc, which CodeGen
already depends on, we can sink the shuffle decoding there as well.

Differential Revision: https://reviews.llvm.org/D77980