[Stream] Fix crash in ConvertSplatConstantsIntoSplats for dense_resource (#24683)

## Description

https://github.com/iree-org/iree/issues/24448 crashes because:

1. the frontend generates `DenseResourceElementsAttr` instead of
`SplatElementsAttr` for models, it satisfies `isSplat()` because the
`isSplat` has a default implementation and the tensor's elements share
the same value(for batchnorm, the weight is 1.0, the mean is 0.0 by
default)
2. the subsequent code attempts to `dyn_cast` the attribute to
`SplatElementsAttr` , which returns `nullptr` and causes a segfault upon
dereference.

## Solution

1. restrict the initial cast to `DenseElementsAttr`. since
`DenseResourceElementsAttr` does not inherit from `DenseElementsAttr`,
the cast will safely fail and we could return `failure()` as expected.
2. retrieve the splat value directly via
`DenseElementsAttr::getSplatValue()` , which is natively supported for
all splat-valued dense attributes

## E2E test

run
https://gist.github.com/hsqStephenZhang/31c9516a5b8788e7036061aae0ff46ba

before the fix, it leads to segfault. after the fix, it exit
successfully.

---------

Signed-off-by: hsqStephenZhang <stephenzhang666666@gmail.com>
Co-authored-by: Artem Gindinson <gindinson@roofline.ai>