Clean up null check handling in EffectAnalyzer (#8422)

Each visitor previously handled implicit null checks of reference
operands in an ad hoc way. Different expressions had varying levels of
precision when reasoning about the null checks. For example, many would
just always set `implicitTraps`, while others would look at the
nullability of the operands to be more precise.

Factor the handling of null checks into a helper function and use it
everywhere applicable. This makes the code smaller, clearer, and more
precise. Update OptimizeCasts to add a new dummy operand to a dummy
expression because EffectAnalyzer will newly try to look at that
operand and it must not read uninitialized memory.

Some GUFA tests are updated to remove more side-effect-free expressions
now that EffectAnalyzer is more aggressive and can recognize them as
such.

Author: tlively