Analyze constants to remove unnecessary branches

This commit adds analysis and optimization of constants to remove
unnecessary branches. It uses abstract interpretation to determine
what constant(s) a particular stack slot or variable might or might not
hold at a given spot and uses that knowledge to eliminate branches that
cannot be taken. Its primary goal is null check removal, but it also
works for other constants.

Several tests are modified to include the new optimization phase.

Two new tests are added. One verifies that branching still works as
expected. The other verifies that branches are removed.