Fix build after flake8 update.

There's a new version of flake8 out which doesn't like variables names
i, l, I.

No functional change intended.

lib/cretonne/src/isa/riscv/mod.rs

@@ -147,7 +147,10 @@ mod tests {
         };
 
         // ADDI is I/0b00100
-        assert_eq!(encstr(&*isa, isa.encode(&dfg, &inst64, types::I64)), "I#04");
+        assert_eq!(
+            encstr(&*isa, isa.encode(&dfg, &inst64, types::I64)),
+            "Ii#04"
+        );
 
         // Try to encode iadd_imm.i64 v1, -10000.
         let inst64_large = InstructionData::BinaryImm {
@@ -167,7 +170,10 @@ mod tests {
         };
 
         // ADDIW is I/0b00110
-        assert_eq!(encstr(&*isa, isa.encode(&dfg, &inst32, types::I32)), "I#06");
+        assert_eq!(
+            encstr(&*isa, isa.encode(&dfg, &inst32, types::I32)),
+            "Ii#06"
+        );
     }
 
     // Same as above, but for RV32.
@@ -211,7 +217,10 @@ mod tests {
         };
 
         // ADDI is I/0b00100
-        assert_eq!(encstr(&*isa, isa.encode(&dfg, &inst32, types::I32)), "I#04");
+        assert_eq!(
+            encstr(&*isa, isa.encode(&dfg, &inst32, types::I32)),
+            "Ii#04"
+        );
 
         // Create an imul.i32 which is encodable in RV32, but only when use_m is true.
         let mul32 = InstructionData::Binary {