[NoTTI] reject negative scale in addressing mode

Summary:
I noticed this bug when deubging a WIP on LSR. I wonder whether and how we
should add a regression test for this.

Test Plan: no tests failed.

Reviewers: atrick

Subscribers: hfinkel, llvm-commits

Differential Revision: http://reviews.llvm.org/D9536

llvm-svn: 236887