[ARM, Thumb1] Prevent ARMTargetLowering::isLegalAddressingMode from accepting illegal modes

ARMTargetLowering::isLegalAddressingMode can accept illegal addressing modes
for the Thumb1 target. This causes generation of redundant code and affects
performance.

This fixes PR34106: https://bugs.llvm.org/show_bug.cgi?id=34106

Differential Revision: https://reviews.llvm.org/D36467

llvm-svn: 311649