[X86] Remove use of MVT class from the ShuffleDecode library.

MVT belongs to the CodeGen layer, but ShuffleDecode is used by the X86 InstPrinter which is part of the MC layer. This only worked because MVT is completely implemented in a header file with no other library dependencies.

Differential Revision: https://reviews.llvm.org/D44353

llvm-svn: 327292