cmd/compile: prevent pointer shaping of array pointer shape types

[alexandre-daubois]

CL 704095 added an exception in shapify to prevent pointer shaping of
*[]go.shape.T, which would lose the original type by converting it to
*go.shape.uint8.

However, there is another edge case when the type argument is itself
already a shape type (e.g., go.shape.*[1]uint8). This happens when a
shaped pointer type from an outer generic function is used to
instantiate an inner generic function with a basic interface constraint.
In this case, pointer shaping converts go.shape.*[1]uint8 to
go.shape.*uint8, losing the array element type. Subsequent slice
operations then fail with "bad ptr to array in slice" because uint8 is
not an array type.

This commit adds an exception to skip pointer shaping when the type
argument is a pointer shape type whose element is an array, preserving
the array type information needed for slice operations.

Fixes #78297

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Alexandre Daubois:

Patch Set 1:

(1 comment)

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Message from Cuong Manh Le:

Patch Set 2: Hold+1

(1 comment)

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[gopherbot]

Message from Alexandre Daubois:

Patch Set 2:

(1 comment)

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This PR (HEAD: fa4dada) has been imported to Gerrit for code review.

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