fixup! mir-opt: Eliminate dead statements even if they are used by debuginfos

Add a test case for a invalid place on debuginfos

Author: dianqk

tests/mir-opt/pre-codegen/dead_on_invalid_place.invalid_place.PreCodegen.after.mir

@@ -0,0 +1,13 @@
+// MIR for `invalid_place` after PreCodegen
+
+fn invalid_place(_1: bool) -> bool {
+    debug c1_ref => _2;
+    let mut _0: bool;
+    let mut _2: &bool;
+
+    bb0: {
+        // DBG: _2 = &?;
+        _0 = copy _1;
+        return;
+    }
+}

tests/mir-opt/pre-codegen/dead_on_invalid_place.rs

@@ -0,0 +1,27 @@
+#![feature(core_intrinsics, custom_mir)]
+#![crate_type = "lib"]
+
+use std::intrinsics::mir::*;
+
+// EMIT_MIR dead_on_invalid_place.invalid_place.PreCodegen.after.mir
+#[custom_mir(dialect = "runtime")]
+pub fn invalid_place(c: bool) -> bool {
+    // CHECK-LABEL: fn invalid_place
+    // CHECK: debug c1_ref => [[c1_ref:_[0-9]+]];
+    // CHECK: bb0: {
+    // We cannot read the reference, since `c1` is dead.
+    // CHECK-NEXT: DBG: [[c1_ref]] = &?
+    // CHECK-NEXT: _0 = copy _1;
+    // CHECK-NEXT: return;
+    mir! {
+        let _c1_ref: &bool;
+        let c1: bool;
+        debug c1_ref => _c1_ref;
+        {
+            c1 = c;
+            _c1_ref = &c1;
+            RET = c;
+            Return()
+        }
+    }
+}