Recommit "[AArch64] Fix incorrect `isLegalAddressingMode`"

This patch recommits 0827e2fa after
reverting it in ed7ada25.  Added
workround for `Targetlowering::AddrMode` no longer being an aggregate
in C++20.

`AArch64TargetLowering::isLegalAddressingMode` has a number of
defects, including accepting an addressing mode, which consists of
only an immediate operand, or not checking the offset range for an
addressing mode in the form `1*ScaledReg + Offs`.

This patch fixes the above issues.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D143895

Change-Id: I41a520c13ce21da503ca45019979bfceb8b648fa