Add invariant to Vec::pop that len < cap if pop successful

Fixes: rust-lang#114334

Author: krtab

alloc/src/vec/mod.rs

@@ -1956,6 +1956,7 @@ impl<T, A: Allocator> Vec<T, A> {
         } else {
             unsafe {
                 self.len -= 1;
+                core::intrinsics::assume(self.len < self.capacity());
                 Some(ptr::read(self.as_ptr().add(self.len())))
             }
         }