[TwoAddressInstructionPass] Special processing of STATEPOINT instruction.

STATEPOINT is a special pseudo instruction which represent Moving GC semantic to LLVM.
Every tied def/use VReg pair in STATEPOINT represent same physical register which can
'magically' change during call wrapped by statepoint.
(By construction, tied use operand  is not live across  STATEPOINT).

This means that when converting into two-address form, there is not need to insert COPY
instruction before stateppoint, what TwoAddressInstruction pass does for 'regular'
instructions.

Reviewed By: MatzeB

Differential Revision: https://reviews.llvm.org/D124631