[Relax] Fix FoldConstant crash on data-dependent reshape with unknown shape

[cchung100m]

Hi Committers,

This PR fixes issues #20260. Any suggestions would be appreciated if you are available.

Root Cause

FoldConstant has a speculative step that calls an op's registered FLegalize function to check if it would fold into an all-constant call_tir. Unlike the real LegalizeOps pass, this speculative probe never checks whether the call's argument/return shapes are actually known before invoking FLegalize.

When reshape's target shape comes from an unfolded tensor_to_shape call (i.e., a genuinely non-constant tensor, so the shape values are unknown), this lets the unresolved shape reach reshape's legalizer (reshape_call_te), which asserts - with no message - that the shape has already resolved to a ShapeExpr. That's what actually crashes.

Note: the original issue attributes the crash to fold_constant.cc's shape_to_tensor special case, but that code path is dead for this repro (shape_to_tensor has had a real FLegalize since #19957) - it was already stale when the issue was filed. This PR fixes the crash reachable today (the tensor_to_shape -> reshape case). The issue's other example (shape_to_tensor fed directly into reshape) is unrelated to this crash path. It is now rejected earlier, at graph-construction time, by unrelated, more recent type-inference changes - not by this PR.

Solutions

• Extract the "can this op be legalized right now" check (previously a private lambda inside LegalizeOps) into a shared CanLegalizedCall helper in src/relax/transform/utils.h, reused by both LegalizeOps and FoldConstant.
• FoldConstant now checks CanLegalizeCall before its speculative legalize-and-fold probe; when shapes aren't statically known, it skips folding and restores the call's original type (previously reset to Type::Missing(), which could let a later stage infer an ill-formed type referencing a block-scoped DataflowVar).
• Hardened reshape/broadcast_to/collapse_sum_*'s shared FLegalize (reshape_call_te) to raise a descriptive ValueError instead of a bare assert, as defense in depth.

[cchung100m]

cc @tlopex @mshr-h