Improve conditional matrix mult with 3 opts (#116)

We received a benchmark from Lef which essentially checked the
following:

   (cond ? A*B : A*C)[i] == v

where A, B, C are 4 by 4 matrices, cond is a bool, i is an index, and v
a value.

The expected number of constraints is n^3: order 64 , but compile time
and constraint count were in the thousands.

I investigated finding a number of things:
* the Z# front-end entered a conditional branch for the ternary, causing
  the MM to happen in a conditional context
* while the MM has an oblivious access pattern, since `i` is variable,
  the oblivious pass wasn't really helping
* there were some weird non-constant indices floating around like
  (ite cond 0 0)

The three optimizations fix these problems:
* If the Z# frontend isn't in assertion isolation mode, don't enter a
  conditional branch for an ITE
* Constant folding: add the rewrite (ite cond a a) -> a
* Linear-scan array eliminator: for each operation, check if the index
  is constant. If so, skip the scan (for that access only)

examples/ZoKrates/pf/mm4_cond.zok

@@ -0,0 +1,18 @@
+def matmult(field[16] a, field[16] b) -> field[16]:
+          field[16] c = [0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0]
+  
+          for field i in 0..4 do
+                  for field j in 0..4 do
+                          field s = 0
+                          for field k in 0..4 do
+                                  s = s + a[i*4 + k] * b[k*4 + j]
+                          endfor
+                  c[i*4 +j] = s
+                  endfor
+          endfor
+          return c
+  
+def main(public field[16] a, public field[16] b, public field[2] ab, public field init, public field final, private field doc) -> bool:
+          field[16] s = [1,0,0,0, 0,1,0,0, 0,0,1,0, 0,0,0,1]
+          s = if (doc == 0) then matmult(s, a) else matmult(s, b) fi
+          return if s[init*4 + final] == 1 then true else false fi