[llvm-dev] How do I prevent DAGCombiner from combining ZExt/SExt(load) into LoadZExt?

[Zhang via llvm-dev]

In our toy ISA, we don't have a dedicated ZExtLoad instruction. However DAGCombiner tends to combine ZERO_EXTEND(LOAD) into a LoadZExt node which can't be trivially selected:


```
Combining: t5: i32 = zero_extend t4
Creating new node: t9: i32,ch = load<(dereferenceable load 1 from %ir.2), zext from i1> t0, FrameIndex:i64<0>, undef:i64

```


Reading through the implementation of ``tryToFoldExtOfLoad`` is still equally confusing
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