The memory access API supports custom alignment constraints, which are checked upon memory access, using the following formula:

((segmentBaseAddress + accessedOffset) & alignmentMask) == 0

However, when accessing a segment using a var handle obtained from a layout featuring a non-trivial alignment mask, access performance is slower than in the case where the alignment mask is 0.

The attached patch adds a benchmark which shows the problem; the benchmark compares accessing a segment using a 4-byte aligned vs. a 1-byte aligned layout:

```
Benchmark Mode Cnt Score Error Units
LoopOverNonConstant.segment_loop_instance_index avgt 30 0.229 ? 0.001 ms/op
LoopOverNonConstant.segment_loop_instance_index_aligned avgt 30 0.329 ? 0.005 ms/op
```

As it can be seen, access with alignment constraints is slower.

This is mildly surprising - after all, in the above formula, segmentBaseAddress is a loop invariant - whereas accessedOffset typically depends on the loop variable, so existing BCE logic should kick in and detect that the offset is always aligned (given the loop stride).