[llvm] [GlobalISel] Combine G_MERGE_VALUES of x and undef (PR #113616)

[Thorsten Schütt via llvm-commits]

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@@ -412,7 +412,7 @@ def binop_right_undef_to_undef: GICombineRule<
 
 def unary_undef_to_zero: GICombineRule<
   (defs root:$root),
-  (match (wip_match_opcode G_ABS):$root,
+  (match (wip_match_opcode G_ABS, G_ZEXT):$root,
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tschuett wrote:

One version of this PR combined into G_ZEXT. Here, we are combining G_ZEXT of undef into 0. It relies on the helper
```
 Helper.replaceInstWithConstant(*${root}, 0); 
```

https://github.com/llvm/llvm-project/pull/113616