Optimization: replace sub vN, vN with zero

[asterite]

While looking at the SSA for this code:

use std::hash::sha256::sha256;

fn main(input: [u8; 32]) -> pub [u8; 32] {
    sha256(input)
}

I found this instruction:

    v172 = sub v41, v41

I think that could be replaced with the constant zero. There are two occurrences of that in the final SSA. Maybe it could also help with constant propagation, etc.

I tried implementing this with this patch:

diff --git a/compiler/noirc_evaluator/src/ssa/ir/instruction/binary.rs b/compiler/noirc_evaluator/src/ssa/ir/instruction/binary.rs
index df1e8f537d..14cddb7f41 100644
--- a/compiler/noirc_evaluator/src/ssa/ir/instruction/binary.rs
+++ b/compiler/noirc_evaluator/src/ssa/ir/instruction/binary.rs
@@ -103,6 +103,11 @@ impl Binary {
             );
         }

+        if self.lhs == self.rhs {
+            let zero = dfg.make_constant(FieldElement::zero(), lhs_type);
+            return SimplifyResult::SimplifiedTo(zero);
+        }
+
         let operator = if lhs_type == NumericType::NativeField {
             // Unchecked operations between fields or bools don't make sense, so we convert those to non-unchecked
             // to reduce noise and confusion in the generated SSA.

It works for a simple program like this one:

fn main(input: Field) -> pub Field {
    input - input
}

(it now returns Field 0 instead of doing the subtraction)

but the compiler enters a loop in the sha256 case and I don't understand why. It seems it happens during unrolling.