ciflow/torchtitan/201029: Keep a zero exact through a low-precision rounding cast

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Rounding a constant zero to float16 or bfloat16 gives exactly zero, so the prologue zero-mask analysis can keep it, as it did before #197132 . A prologue whose only low-precision op is a rounding cast then keeps its masked zeros and gets no extra tl.where, matching 2.14's generated code. Other values still become unknown. The end-to-end test now checks the generated code for the mask, the same way test_upcast does.

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