[mlir][tosa] Fix assertion failure in tosa-layerwise-constant-fold (#85670)

The existing implementation of tosa-layerwise-constant-fold only works
for constant values backed by DenseElementsAttr. For constants which
hold DenseResourceAttrs, the folder will end up asserting at runtime, as
it assumes that the backing data can always be accessed through
ElementsAttr::getValues.

This change reworks the logic so that types types used to perform
folding are based on whether the ElementsAttr can be converted to a
range of that particular type.

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Co-authored-by: Spenser Bauman <sabauma@mathworks.com>
Co-authored-by: Tina Jung <tinamaria.jung@amd.com>