Precommit test for rust-lang#118328.

Author: aDotInTheVoid

tests/mir-opt/const_prop/issue_118328.rs

@@ -0,0 +1,20 @@
+// unit-test: ConstProp
+// compile-flags: -O
+// skip-filecheck
+#![allow(unused_assignments)]
+
+struct SizeOfConst<T>(std::marker::PhantomData<T>);
+impl<T> SizeOfConst<T> {
+    const SIZE: usize = std::mem::size_of::<T>();
+}
+
+// EMIT_MIR issue_118328.size_of.ConstProp.diff
+fn size_of<T>() -> usize {
+    let mut a = 0;
+    a = SizeOfConst::<T>::SIZE;
+    a
+}
+
+fn main() {
+    assert_eq!(size_of::<u32>(), std::mem::size_of::<u32>());
+}

tests/mir-opt/const_prop/issue_118328.size_of.ConstProp.diff

@@ -0,0 +1,21 @@
+- // MIR for `size_of` before ConstProp
++ // MIR for `size_of` after ConstProp
+  
+  fn size_of() -> usize {
+      let mut _0: usize;
+      let mut _1: usize;
+      scope 1 {
+          debug a => _1;
+      }
+  
+      bb0: {
+          StorageLive(_1);
+          _1 = const 0_usize;
+          _1 = const _;
+-         _0 = _1;
++         _0 = const 0_usize;
+          StorageDead(_1);
+          return;
+      }
+  }
+