[AArch64] NFC: Cleanup isAArch64FrameOffsetLegal

Cleanup isAArch64FrameOffsetLegal by:
- Merging the large switch statement to reuse AArch64InstrInfo::getMemOpInfo().
- Using AArch64InstrInfo::getUnscaledLdSt() to determine whether an instruction
  has an unscaled variant.
- Simplifying the logic that calculates the offset to fit the immediate.

Reviewers: paquette, evandro, eli.friedman, efriedma

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D59636

llvm-svn: 357064