LLVM: fix missing alignment on wrapping instructions

Previously, when lowering AIR instructions `wrap_errunion_payload`,
`wrap_errunion_err`, and `wrap_optional`, the LLVM backend would create
an alloca instruction to store the result, but did not set the alignment
on it. This caused UB which went undetected for a long time until we
started enabling the stack protector.

Closes #12594
Unblocks #12508
Inspires #12634

Tests passed locally:
 * test-behavior
 * test-cases

1 files changed, 16 insertions, 0 deletions

diff --git a/test/behavior/error.zig b/test/behavior/error.zig
index b355c85819..684b01a797 100644
--- a/test/behavior/error.zig
+++ b/test/behavior/error.zig
@@ -796,3 +796,19 @@ test "error union of noreturn used with catch" {
     const err = NoReturn.testCatch();
     try expect(err == error.OtherFailure);
 }
+
+test "alignment of wrapping an error union payload" {
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
+
+    const S = struct {
+        const I = extern struct { x: i128 };
+
+        fn foo() anyerror!I {
+            var i: I = .{ .x = 1234 };
+            return i;
+        }
+    };
+    try expect((S.foo() catch unreachable).x == 1234);
+}