Move /src/test to /tests

tests/mir-opt/issues/issue_75439.rs

@@ -0,0 +1,18 @@
+// EMIT_MIR issue_75439.foo.MatchBranchSimplification.diff
+
+use std::mem::transmute;
+
+pub fn foo(bytes: [u8; 16]) -> Option<[u8; 4]> {
+    // big endian `u32`s
+    let dwords: [u32; 4] = unsafe { transmute(bytes) };
+    const FF: u32 = 0x0000_ffff_u32.to_be();
+    if let [0, 0, 0 | FF, ip] = dwords {
+        Some(unsafe { transmute(ip) })
+    } else {
+        None
+    }
+}
+
+fn main() {
+    let _ = foo([0; 16]);
+}