[AVX] Recorganize X86ShuffleDecode into its own library
(LLVMX86Utils.a) to break cyclic library dependencies between
LLVMX86CodeGen.a and LLVMX86AsmParser.a.  Previously this code was in
a header file and marked static but AVX requires some additional
functionality here that won't be used by all clients.  Since including
unused static functions causes a gcc compiler warning, keeping it as a
header would break builds that use -Werror.  Putting this in its own
library solves both problems at once.

llvm-svn: 125765