Update pattern-matching for error unwrapping

The new .save_err_ret_index that can be at the front of a block can
surprise the `catch unreachable` pattern matcher.

src/Sema.zig

@@ -10668,6 +10668,7 @@ fn maybeErrorUnwrap(sema: *Sema, block: *Block, body: []const Zir.Inst.Index, op
     const tags = sema.code.instructions.items(.tag);
     for (body) |inst| {
         switch (tags[inst]) {
+            .save_err_ret_index,
             .dbg_block_begin,
             .dbg_block_end,
             .dbg_stmt,
@@ -10690,6 +10691,10 @@ fn maybeErrorUnwrap(sema: *Sema, block: *Block, body: []const Zir.Inst.Index, op
                 try sema.zirDbgStmt(block, inst);
                 continue;
             },
+            .save_err_ret_index => {
+                try sema.zirSaveErrRetIndex(block, inst);
+                continue;
+            },
             .str => try sema.zirStr(block, inst),
             .as_node => try sema.zirAsNode(block, inst),
             .field_val => try sema.zirFieldVal(block, inst),