SSA Transform useless phis on exit block

[dibyendumajumdar]

This test case:

func foo(x: Int, y: Int)->Int {
   var sum: Int
   
   if (x >= y)
      return 0
      
   sum = 0;
   while (x < y) {
      if (x / 2 * 2 == x) {
         sum = sum + 1
      }
      x = x + 1
   }
   return sum
}

Generates:

After SSA
=========
L0:
    arg x_0
    arg y_0
    %t3_0 = x_0>=y_0
    if %t3_0 goto L2 else goto L3
L2:
    ret 0
    goto  L1
L1:
    sum_4 = phi(sum_1, sum_1)
    x_3 = phi(x_1, x_1)
L3:
    sum_0 = 0
    goto  L4
L4:
    sum_1 = phi(sum_0, sum_3)
    x_1 = phi(x_0, x_2)
    %t4_0 = x_1<y_0
    if %t4_0 goto L5 else goto L6
L5:
    %t5_0 = x_1/2
    %t6_0 = %t5_0*2
    %t7_0 = %t6_0==x_1
    if %t7_0 goto L7 else goto L8
L7:
    %t8_0 = sum_1+1
    sum_2 = %t8_0
    goto  L8
L8:
    sum_3 = phi(sum_1, sum_2)
    %t9_0 = x_1+1
    x_2 = %t9_0
    goto  L4
L6:
    ret sum_1
    goto  L1