This patch removes the variants of DecodeVPERMVMask and
DecodeVPERMV3Mask that take "const Constant *C" as they are not used
They were introduced on Sep 8, 2015 in commit
The last use of DecodeVPERMVMask(const Constant *C, ...) was removed
on Feb 7, 2016 in commit 73fc26b44a8591b15f13eaffef17e67161c69388.
The last use of DecodeVPERMV3Mask(const Constant *C, ...) was removed
on May 28, 2018 in commit dcfcfdb0d166fff8388bdd2edc5a2948054c9da1.