[AlignmentFromAssumptions] Don't crash just because the target is 32-bit

We used to crash processing any relevant @llvm.assume on a 32-bit target
(because we'd ask SE to subtract expressions of differing types). I've copied
our 'simple.ll' test, but with the data layout from arm-linux-gnueabihf to get
some meaningful test coverage here.

llvm-svn: 217574