[AArch64] Make getInstSizeInBytes() use instruction size from InstrInfo.td

Currently, AArch64InstrInfo::getInstSizeInBytes() uses hard-coded
instruction size for some pseudo-instructions, while this
information should ideally be found in AArch64InstrInfo.td file (which
can be accessed via MCInstrDesc). Hence, the .td file should be updated
and no hard-coded instruction sizes should be used by
getInstSizeInBytes() anymore.

Differential Revision: https://reviews.llvm.org/D117970