[Assignment Tracking] Remove overly defensive AllocaInst assertion

Remove assert from AssignmentTrackingAnalysis that fires if a local variable
has non-alloca storage. The analysis can emit these locations but the
assignment tracking code in SelectionDAG isn't ready to handle non-alloca
storage for locals yet. The AssignmentTrackingPass (pass that adds assignment
tracking metadata) ignores non-alloca dbg.declares, so the only variables
affected are those who's backing storage is changed from an alloca during
optimisation, and the result is the variables are dropped.

Fixes: https://ci.chromium.org/ui/p/pigweed/builders/toolchain/
                 toolchain-ci-pigweed-linux/b8783274592206481489/overview

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D149135