Decluster some must-be-constant ops to reduce XLA recompilations

The CL is organized as follows:

 - The main change is in jit/partially_decluster_pass.
 - tf2xla/const_analysis now takes an "edge_filter" to facilitate use by
   jit/partially_decluster_pass.
 - tests/dense_layer_test.py was using the execution of ListDiff as what I
   assume is a sanity check to see that the XLA cluster ran.  With this CL the
   ListDiff op gets declustered so we now check for "MatMult" for the sanity
   check.
 - Some tests were dropping TF_XLA_FLAGS; fixed them to not do so.

PiperOrigin-RevId: 212071118