[TVMScript][Relax] Use tir.SizeVar for shape variables

[Lunderberg]

As reported in #16877, shape inference performed during a Relax transformation may produce different results than shape inference performed during TVMScript parsing. While Relax transformations call Analyzer::MarkGlobalNonNegValue for each shape expression, this is not provided during TVMScript parsing. As a result, output shapes that are conditional on the sign of a variable may produce different results when simplifying.

This commit provides a partial resolution for this issue. Where prior to this commit, the TVMScript parser generated a tir.Var for each symbolic variable, the TVMScript parser now provides a tir.SizeVar for symbolic variables that are defined in contexts that require a non-negative size.