Liveness: do not elide safety-checked instructions

Resolves: #19012

src/Air.zig

@@ -1646,20 +1646,20 @@ pub fn mustLower(air: Air, inst: Air.Inst.Index, ip: *const InternPool) bool {
         .c_va_copy,
         .c_va_end,
         .c_va_start,
+        .add_safe,
+        .sub_safe,
+        .mul_safe,
         => true,
 
         .add,
-        .add_safe,
         .add_optimized,
         .add_wrap,
         .add_sat,
         .sub,
-        .sub_safe,
         .sub_optimized,
         .sub_wrap,
         .sub_sat,
         .mul,
-        .mul_safe,
         .mul_optimized,
         .mul_wrap,
         .mul_sat,

test/cases/safety/ignored expression integer overflow.zig

@@ -0,0 +1,21 @@
+const std = @import("std");
+
+pub fn panic(message: []const u8, stack_trace: ?*std.builtin.StackTrace, _: ?usize) noreturn {
+    _ = stack_trace;
+    if (std.mem.eql(u8, message, "integer overflow")) {
+        std.process.exit(0);
+    }
+    std.process.exit(1);
+}
+
+pub fn main() !void {
+    var x: usize = undefined;
+    x = 0;
+    // We ignore this result but it should still trigger a safety panic!
+    _ = x - 1;
+    return error.TestFailed;
+}
+
+// run
+// backend=llvm
+// target=native