Rollup merge of rust-lang#82874 - erikdesjardins:cgtests, r=nagisa

Add codegen tests for some issues closed by LLVM 12

Namely rust-lang#73031, rust-lang#75546, and rust-lang#77812

Author: m-ou-se

src/test/codegen/issue-73031.rs

@@ -0,0 +1,27 @@
+// min-llvm-version: 12.0.0
+// compile-flags: -O
+#![crate_type = "lib"]
+
+// Test that LLVM can eliminate the unreachable `All::None` branch.
+
+pub enum All {
+    None,
+    Foo,
+    Bar,
+}
+
+// CHECK-LABEL: @issue_73031
+#[no_mangle]
+pub fn issue_73031(a: &mut All, q: i32) -> i32 {
+    *a = if q == 5 {
+        All::Foo
+    } else {
+        All::Bar
+    };
+    match *a {
+        // CHECK-NOT: panic
+        All::None => panic!(),
+        All::Foo => 1,
+        All::Bar => 2,
+    }
+}

src/test/codegen/issue-75546.rs

@@ -0,0 +1,16 @@
+// min-llvm-version: 12.0.0
+// compile-flags: -O
+#![crate_type = "lib"]
+
+// Test that LLVM can eliminate the impossible `i == 0` check.
+
+// CHECK-LABEL: @issue_75546
+#[no_mangle]
+pub fn issue_75546() {
+    let mut i = 1u32;
+    while i < u32::MAX {
+        // CHECK-NOT: panic
+        if i == 0 { panic!(); }
+        i += 1;
+    }
+}

src/test/codegen/issue-77812.rs

@@ -0,0 +1,33 @@
+// min-llvm-version: 12.0.0
+// compile-flags: -O
+#![crate_type = "lib"]
+
+// Test that LLVM can eliminate the unreachable `Variant::Zero` branch.
+
+#[derive(Copy, Clone, Eq, PartialEq)]
+pub enum Variant {
+    Zero,
+    One,
+    Two,
+}
+
+extern {
+    fn exf1();
+    fn exf2();
+}
+
+pub static mut GLOBAL: Variant = Variant::Zero;
+
+// CHECK-LABEL: @issue_77812
+#[no_mangle]
+pub unsafe fn issue_77812() {
+    let g = GLOBAL;
+    if g != Variant::Zero {
+        match g {
+            Variant::One => exf1(),
+            Variant::Two => exf2(),
+            // CHECK-NOT: panic
+            Variant::Zero => panic!(),
+        }
+    }
+}