This patch abstracts out a small part of simplify_comparison(), into a
new function simplify_compare_const(). The parts are basically where the
comparison RTX code and a constant operand (op1) can be simplified
towards "compare with zero". Note this part leaves op0 untouched, thus
not passed into simplify_compare_const() by reference.

By itself this patch should only be a code reorganization, and changes
nothing to the compiler.

This is part of a small improvement to the compare combining code in the
later patch. During before, the entire case required operand[1] of i3 to
be const0_rtx, now we can try a little tweaking of the code and
constant, and see if it then changes to a cmp zero.

This abstracted patch was bootstrapped and tested independently on i686,
to be sure it was only a reorg.