[ARM] Fix PR37382: Don't optimize mul.with.overflow on thumbv6m.

Reviewers: efriedma, rogfer01, javed.absar

Reviewed By: efriedma, rogfer01

Subscribers: kristof.beyls, chrib, llvm-commits

Differential Revision: https://reviews.llvm.org/D48846

llvm-svn: 336144