[DAGCombiner] Support non-zero offset in load combine

[apilipenko]

Summary

Enable folding patterns which load the value from non-zero offset:

i8 *a = ...
i32 val = a[4] | (a[5] << 8) | (a[6] << 16) | (a[7] << 24)

>

i32 val = *((i32*)(a+4))

[boris.ulasevich]

test/CodeGen/AArch64/load-combine-big-endian.ll

203 ↗

(On Diff #86624)

Just a question.. This inefficient implementation should be optimised in spite of non-zero offset. How do we check the optimisation works Ok?

[RKSimon]

Please can you commit the new tests to trunk now with their current codegen and then update this patch to show the delta in codegen - I think this will answer @boris.ulasevich 's concerns.

[apilipenko]

The test cases have been landed separately (rL294185), rebase the patch.

[RKSimon]

One minor

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

4596 ↗

(On Diff #87222)

Add an assert that ensures FirstOffset has a sane value.

[apilipenko]

Add an assert as suggested.

[RKSimon]

LGTM