[AArch64] Fix incorrect `isLegalAddressingMode`

`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: I756fa21941844ded44f082ac7eea4391219f9851