[dfsan] Remove hardcoded shadow width in struct.ll

As a preparation step for fast8 support, we need to update the tests
to pass in both modes. That requires generalizing the shadow width
and remove any hard coded references that assume it's always 2 bytes.

Reviewed By: stephan.yichao.zhao

Differential Revision: https://reviews.llvm.org/D98090

Author: gbalats