//
//	   b        b           b       b
//	\ / \ /    | \  /    \ / |     | |
//	 s0  s1    |  s1      s0 |     | |
//	  \ /      | /         \ |     | |
//	   ss      ss           ss      ss
//
// If all Phi ops in ss have identical variables for slots corresponding to
// s0, s1 and b then the branch can be dropped.
// This optimization often comes up in switch statements with multiple
// expressions in a case clause:
//