MemorySSA: Fix example in header comment.

[pcc]

Summary

This fixes the example so that it matches the pass's behavior. I was a
little confused by the example until I tried running it and realized that
there was a mistake.

[pcc]

• Another correction

[• dberlin]

Lgtm.

include/llvm/Transforms/Utils/MemorySSA.h

 //   %2 = bitcast i8* %call to i32*
 //   ; 2 = MemoryDef(1)
 //   %call1 = call noalias i8* @_Znwm(i64 4) #3
 //   %4 = bitcast i8* %call1 to i32*
 //   ; 3 = MemoryDef(2)
 //   store i32 5, i32* %2, align 4
 //   ; 4 = MemoryDef(3)
 //   store i32 7, i32* %4, align 4
-//   ; MemoryUse(4)
-//   %7 = load i32* %2, align 4
 //   ; MemoryUse(3)
+//   %7 = load i32* %2, align 4
+//   ; MemoryUse(4)
 //   %8 = load i32* %4, align 4
 //   %add = add nsw i32 %7, %8
 //   ret i32 %add
 // }
 //
 // Given this form, all the stores that could ever effect the load at %8 can be
 // gotten by using the MemoryUse associated with it, and walking from use to def
 // until you hit the top of the function.