[llvm] [GlobalISel] Document minimum legality requirements for G_IMPLICIT_DEF. (PR #117609)

[Amara Emerson via llvm-commits]

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@@ -338,8 +338,8 @@ G_BUILD_VECTOR_TRUNC, G_CONCAT_VECTORS, G_UNMERGE_VALUES, G_PTRTOINT, and
 G_INTTOPTR have already been noted above. In addition to those, the following
 operations have requirements:
 
-* At least one G_IMPLICIT_DEF must be legal. This is usually trivial as it
-  requires no code to be selected.
+* For every type that can be produced by any instruction, G_IMPLICIT_DEF must be
+  legal. This is usually trivial as it requires no code to be selected.
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aemerson wrote:

> That could create illegal MIR after the legalizer. Please tell me when you finished inspecting all downstream legalizers.

So two things here:
1) We are requiring that targets make something legal. It's down to the targets to ensure this happens, not generic GlobalISel code.
2) Even if the first point didn't exist, which particular downstream legalizers do you want me to look at?

https://github.com/llvm/llvm-project/pull/117609