Zimm16 is now dead. Its entry is not removed from the enum, to avoid having

Zimm16 is now dead.  Its entry is not removed from the enum, to avoid having
to renumber everything.  Similar elimination should be applied to other
operand enum values that are only used to format printing in the .s file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@15755 91177308-0d34-0410-b5e6-96231b3b80d8