TableGen: Support target specialized pseudoinstructions (#159880)

Allow a target to steal the definition of a generic pseudoinstruction
and remap the operands. This works by defining a new instruction, which
will simply swap out the emitted entry in the InstrInfo table.

This is intended to eliminate the C++ half of the implementation
of PointerLikeRegClass. With RegClassByHwMode, the remaining usecase
for PointerLikeRegClass are the common codegen pseudoinstructions.
Every target maintains its own copy of the generic pseudo operand
definitions anyway, so we can stub out the register operands with
an appropriate class instead of waiting for runtime resolution.

In the future we could probably take this a bit further. For example,
there is a similar problem for ADJCALLSTACKUP/DOWN since they depend
on target register definitions for the stack pointer register.