[mlir] detensorize: don't accidentally convert function entry blocks

[mlir] detensorize: don't accidentally convert function entry blocks

In the Linalg detensorize pass, dialect conversion could accidentally
trigger signature conversion of the function entry block after inlining
the body of a Linalg generic into it. Such a conversion is not desirable
because it would break the internal validity of the function op, that is
futhermore not supposed to be detensorized at the boundary. Mitigate
this by creating a dummy (empty) entry block so Linalg operations are
never inlined into it and the conversion is never triggered.

Closes #62249.

Reviewed By: silvas

Differential Revision: https://reviews.llvm.org/D148983