Iterate on the operations to fold in TestConstantFold in reverse to remove...

    Iterate on the operations to fold in TestConstantFold in reverse to remove the need for ConstantFoldHelper to have a flag for insertion at the head of the entry block. This also fixes an asan bug in TestConstantFold due to the iteration order of operations and ConstantFoldHelper's constant insertion placement.

    Note: This now means that we cannot fold chains of operations, i.e. where constant foldable operations feed into each other. Given that this is a testing pass solely for constant folding, this isn't really something that we want anyways. Constant fold tests should be simple and direct, with more advanced folding/feeding being tested with the canonicalizer.

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PiperOrigin-RevId: 242011744