Fix TBAA metadata for loads/stores with negative indices

[abadams]

A predicated vector load with a negative stride is codegen'd as a dense load of the flipped ramp followed by a shuffle. If the load reaches the first element of the buffer, the flipped ramp starts before it, and the masked-off lanes give the ramp a negative base. add_tbaa_metadata asserted that the base was non-negative.

The TBAA tree describes power-of-two-sized blocks aligned relative to the start of the buffer, so it can't name a range that straddles the start. Emit metadata only down to the buffer level in that case.

Found while working on a fuzzer for sliding window schedules, but not actually related to sliding window.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.75%. Comparing base (26c31e1) to head (b898931).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/CodeGen_LLVM.cpp	75.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9310      +/-   ##
==========================================
- Coverage   69.79%   69.75%   -0.04%     
==========================================
  Files         258      258              
  Lines       77956    77960       +4     
  Branches    18979    18980       +1     
==========================================
- Hits        54410    54383      -27     
+ Misses      17803    17795       -8     
- Partials     5743     5782      +39     

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