[Assignment Tracking] Fix fragments for assignments to variables smaller than the alloca

Prior to this patch the trackAssignments function would attribute all stores to
an alloca to all variables linked to the alloca. This is wrong in the case
where the alloca contains variables which are smaller than the alloca, and
caused erroneous fragment information to be generated.

Now stores outside the variable bounds are discarded, and we check whether a
fragment is needed based on whether the store covers the entire variable as
opposed to whether it covers the entire alloca (except for variables of unknown
size).

Note that trackAssignments doesn't yet understand whole variables sitting at
anything other than offset 0 in an alloca - those variables are still tracked
using dbg.declares.

Fixes https://lab.llvm.org/buildbot/#/builders/70/builds/36007

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D147777