Improve LinearValidOnShape.

It actually only needs to check that the operation is a bitcast (ignoring
element_type). So far, the check was more restrictive, which made this function
always return false for a non-trivial reshape operation.
However we still fail to make use of this less strict checking, because for
reshapes inside a fusion node, we don't have a layout and can therefore not
check if it is a bitcast or not.
Also add a disabled test that will be enabled once the layout issue is fixed.

PiperOrigin-RevId: 187860440