[PCF] Add tensor.collapse_shape fusion into pcf.generic/loop (#23491)

Fuse tensor.collapse_shape consumers into producer pcf.generic/pcf.loop
ops. This supports all possible dynamic cases by emitting loops along
outer dimensions, effectively unrolling the producer `pcf.write_slice`
ops allowing for continued fusion. The alternative to this (essentially
premature unrolling) is to put the reshape on the sref, though to
continue fusing we still would have to do the same (or propagate the
collapse, which should have already been attempted by this point).

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: qedawkins