Answer from cs61c-ae (Zhibo Chen 15283355) for Question 1
As far as MIPS is concerned, only the if and goto statements are required.  Both the do while and the while statements can be performed with just if statements and gotos.  I believe that the two while statements are included to make life easier for C programmers since doing a while loop with if statements is not fun.  Also, C doesn't seem to have the goto option that MIPS does.