chore: bump rust version to 1.71 (FuelLabs#4805)

## Description

Rust v1.71 is
[released](https://blog.rust-lang.org/2023/07/13/Rust-1.71.0.html). This
PR bumps rust version used in CI and fixes warnings returned from
clippy's new version.

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ concurrency:
 env:
   CARGO_TERM_COLOR: always
   REGISTRY: ghcr.io
-  RUST_VERSION: 1.70.0
+  RUST_VERSION: 1.71.0
   NIGHTLY_RUST_VERSION: nightly-2023-02-08
 
 jobs:

sway-ir/src/function.rs

@@ -513,8 +513,7 @@ impl Function {
         let mut res = format!("digraph {} {{\n", self.get_name(context));
 
         worklist.push(entry);
-        while !worklist.is_empty() {
-            let n = worklist.pop().unwrap();
+        while let Some(n) = worklist.pop() {
             visited.insert(n);
             for BranchToWithArgs { block: n_succ, .. } in n.successors(context) {
                 let _ = writeln!(

sway-ir/src/optimize/dce.rs

@@ -254,9 +254,7 @@ pub fn dce(
 
     let mut modified = false;
     let mut cemetery = FxHashSet::default();
-    while !worklist.is_empty() {
-        let dead = worklist.pop().unwrap();
-
+    while let Some(dead) = worklist.pop() {
         if !can_eliminate_instruction(context, dead, &num_symbol_uses, escaped_symbols)
             || cemetery.contains(&dead)
         {

sway-ir/src/optimize/mem2reg.rs

@@ -176,8 +176,7 @@ pub fn promote_to_registers(
         }
     }
     // Transitively add PHIs, till nothing more to do.
-    while !worklist.is_empty() {
-        let (local, ty, known_def) = worklist.pop().unwrap();
+    while let Some((local, ty, known_def)) = worklist.pop() {
         for df in dom_fronts[&known_def].iter() {
             if !new_phi_tracker.contains(&(local.clone(), *df)) && liveins[df].contains(&local) {
                 // Insert PHI for this local at block df.

sway-ir/src/optimize/memcpyopt.rs

@@ -708,8 +708,7 @@ fn is_clobbered(
     let mut worklist: Vec<(Block, Box<dyn Iterator<Item = Value>>)> =
         vec![(store_block, Box::new(iter))];
     let mut visited = FxHashSet::default();
-    'next_job: while !worklist.is_empty() {
-        let (block, iter) = worklist.pop().unwrap();
+    'next_job: while let Some((block, iter)) = worklist.pop() {
         visited.insert(block);
         for inst in iter {
             if inst == load_val || inst == store_val {

sway-ir/src/optimize/simplify_cfg.rs

@@ -109,8 +109,7 @@ fn remove_dead_blocks(context: &mut Context, function: &Function) -> Result<bool
     worklist.push(entry_block);
 
     // Mark reachable nodes.
-    while !worklist.is_empty() {
-        let block = worklist.pop().unwrap();
+    while let Some(block) = worklist.pop() {
         let succs = block.successors(context);
         for BranchToWithArgs { block: succ, .. } in succs {
             // If this isn't already marked reachable, we mark it and add to the worklist.