[llvm] [GlobalISel] Introduce `G_POISON` (PR #127825)

[Matt Arsenault via llvm-commits]

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@@ -430,6 +430,12 @@ def binop_right_undef_to_undef: GICombineRule<
          [{ return Helper.matchOperandIsUndef(*${root}, 2); }]),
   (apply [{ Helper.replaceInstWithUndef(*${root}); }])>;
 
+def binop_right_poison_to_poison
+    : GICombineRule<(defs root:$root),
+                    (match(wip_match_opcode G_SHL, G_ASHR, G_LSHR):$root,
+                        [{ return Helper.matchOperandIsPoison(*${root}, 2); }]),
+                    (apply [{ Helper.replaceInstWithPoison(*${root}); }])>;
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arsenm wrote:

I would drop all of the combine handling in the initial patch. There needs to be more fusion with undef

https://github.com/llvm/llvm-project/pull/127825