Implement op2_range for pointer_plus to determine the offset (operand 2) 
is zero or non-zero based on equality/inequality between the LHS and op1.

Fairly trivial fix for the PR, dependent on the first patch in the set 
as it uses an accurate relation_trio in GORI to determine if LHS == OP1.


There was one tweak to GORI in that we use to stop calculating when the 
LHS was varying.  THis PR also exposed a case where the LHS is varying, 
but a relation between the operands can still cause us to find a useful 
result...  ie

VARYING = VARYING + OFFSET when LHS and OP1 are equal can produce a 
non-zero OFFSET calculation.

Bootstraps on x86_64-pc-linux-gnu with no regressions. OK for trunk?

Andrew