[MC] Do not copy MCInstrDescs. NFC.

Avoid copying MCInstrDesc instances because a future patch will change
them to find their implicit operands and operand info array based on
their own "this" pointer, so it will only work for MCInstrDescs in the
TargetInsts table, not for a copy of an MCInstrDesc at a different
address.

Differential Revision: https://reviews.llvm.org/D142214