Check if debugVar for is debuggable types in the legalization pass (#8326)

## Problem
When generic functions with debug variables were specialized with
concrete types containing non-debuggable fields (e.g.,
`StructuredBuffer`), the IR cloning process would create invalid
`DebugVar` instructions without checking if the substituted types
remained debuggable.

## Solution
This fix adds a defensive check in the legalization pass that removes
the debugVar created for the non-debuggable types.

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Co-authored-by: slangbot <ellieh+slangbot@nvidia.com>
Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

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