[Inductor] Fix constants handling for Triton constexpr (triton#8248)

[jjvraw]

Up until recently (triton-lang/triton#8248), Triton did not explicitly interpret entries in ASTSource.constants , and Inductor historically treated "non-tensor-like" arguments as constexpr, including runtime scalars. Triton’s updated constexpr handling now requires stricter semantics, so this change restricts constants to arguments marked is_constexpr rather than all “non-tensor-like” arguments.

• Fixes issue #170049.

[pytorch-bot[bot]]

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[jjvraw]

@pytorchbot label "topic: not user facing"

[Lucaskabela]

cc @zou3519 for additional review

[Lucaskabela]

Thanks for feedback - this lgtm now but I will defer to Richard for review