[X86] Remove DecodeVPERMVMask and DecodeVPERMV3Mask

This patch removes the variants of DecodeVPERMVMask and
DecodeVPERMV3Mask that take "const Constant *C" as they are not used
anymore.

They were introduced on Sep 8, 2015 in commit
e88038f2.

The last use of DecodeVPERMVMask(const Constant *C, ...)  was removed
on Feb 7, 2016 in commit 73fc26b4.

The last use of DecodeVPERMV3Mask(const Constant *C, ...) was removed
on May 28, 2018 in commit dcfcfdb0.

Differential Revision: https://reviews.llvm.org/D91926