storage_live: avoid computing the layout unless necessary

Author: RalfJung

tests/fail/type-too-large.rs

@@ -1,6 +1,6 @@
 //@ignore-32bit
 
 fn main() {
-    let _fat: [u8; (1 << 61) + (1 << 31)]; //~ ERROR: post-monomorphization error
-    _fat = [0; (1u64 << 61) as usize + (1u64 << 31) as usize];
+    let _fat: [u8; (1 << 61) + (1 << 31)]; // ideally we'd error here, but we avoid computing the layout until absolutely necessary
+    _fat = [0; (1u64 << 61) as usize + (1u64 << 31) as usize]; //~ ERROR: post-monomorphization error
 }

tests/fail/type-too-large.stderr

@@ -1,8 +1,8 @@
 error: post-monomorphization error: values of the type `[u8; 2305843011361177600]` are too big for the current architecture
   --> $DIR/type-too-large.rs:LL:CC
    |
-LL |     let _fat: [u8; (1 << 61) + (1 << 31)];
-   |         ^^^^ values of the type `[u8; 2305843011361177600]` are too big for the current architecture
+LL |     _fat = [0; (1u64 << 61) as usize + (1u64 << 31) as usize];
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ values of the type `[u8; 2305843011361177600]` are too big for the current architecture
    |
    = note: inside `main` at $DIR/type-too-large.rs:LL:CC