[AArch64][SelectionDAG] Supports unpklo/hi instructions to reduce the number of loads

Trying to reduce the number of masked loads in favour of more unpklo/hi
instructions. Both ISD::ZEXTLOAD and ISD::SEXTLOAD are supported to extensions
from legal types.

Both of normal and masked loads test cases added to guard compile crash.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D120953