Fix not complete let before expr in condition

Example
---
```rust
fn f() {
    if $0foo.bar() {}
}
```

**Before this PR**

"let" not in completion list

**After this PR**

```rust
fn f() {
    if let $1 = $0foo.bar() {}
}
```

src/tools/rust-analyzer/crates/ide-completion/src/completions/postfix.rs

@@ -460,6 +460,8 @@ pub(crate) fn is_in_condition(it: &ast::Expr) -> bool {
                 ast::MatchGuard(guard) => guard.condition()? == *it,
                 ast::BinExpr(bin_expr) => (bin_expr.op_token()?.kind() == T![&&])
                     .then(|| is_in_condition(&bin_expr.into()))?,
+                ast::Expr(expr) => (expr.syntax().text_range().start() == it.syntax().text_range().start())
+                    .then(|| is_in_condition(&expr))?,
                 _ => return None,
             } })
         })

src/tools/rust-analyzer/crates/ide-completion/src/tests/expression.rs

@@ -3268,6 +3268,12 @@ fn foo() -> (i32, i32) {
 #[test]
 fn let_in_condition() {
     check_edit("let", r#"fn f() { if $0 {} }"#, r#"fn f() { if let $1 = $0 {} }"#);
+    check_edit("let", r#"fn f() { if $0x {} }"#, r#"fn f() { if let $1 = $0x {} }"#);
+    check_edit(
+        "let",
+        r#"fn f() { if $0foo.bar() {} }"#,
+        r#"fn f() { if let $1 = $0foo.bar() {} }"#,
+    );
 }
 
 #[test]