[Assignment Tracking] Fix fragment error for some DSE-shortened stores

`shortenAssignment` inserts dbg.assigns with fragments describing the dead part
of a shortened store after each dbg.assign linked to the store.

Without this patch it doesn't take into account that the dead part of a
shortened store may be outside the bounds of a variable of a linked
dbg.assign. It also doesn't correctly account for a non-zero offset in the
address modifying `DIExpression` of the dbg.assign (which is possible for
fragments now even though whole variables currently cannot have a non-zero
offset in their alloca).

Fix this by moving the dead slice into variable-space and performing an
intersect of that adjusted slice with the existing fragment.

This fixes a verifier error reported when building fuchsia with assignment
tracking enabled:
https://ci.chromium.org/ui/p/fuchsia/builders/ci/
        clang_toolchain.ci.core.x64-release/b8784000953022145169/overview

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D148536