Add test for cast optimization.

tests/codegen/cast-optimized.rs

@@ -0,0 +1,33 @@
+//@ compile-flags: -O -Z merge-functions=disabled
+#![crate_type = "lib"]
+
+// This tests that LLVM can optimize based on the niches in the source or
+// destination types for casts.
+
+// CHECK-LABEL: @u32_index
+#[no_mangle]
+pub fn u32_index(c: u32) -> [bool; 10] {
+    let mut array = [false; 10];
+
+    let index = (c | 1).leading_zeros() as usize / 4 - 2;
+
+    // CHECK: call core::panicking::panic
+    array[index as usize] = true;
+
+    array
+}
+
+// CHECK-LABEL: @char_as_u32_index
+#[no_mangle]
+pub fn char_as_u32_index(c: char) -> [bool; 10] {
+    let c = c as u32;
+
+    let mut array = [false; 10];
+
+    let index = (c | 1).leading_zeros() as usize / 4 - 2;
+
+    // CHECK-NOT: call core::panicking::panic
+    array[index as usize] = true;
+
+    array
+}