Eliminate excessive null-checks from slice iterators

The data pointer used in the slice is never null, using assume() to tell
LLVM about it gets rid of various unneeded null checks when iterating
over the slice.

Since the snapshot compiler is still using an older LLVM version, omit
the call in stage0, because compile times explode otherwise.

Benchmarks from #18193
````
running 5 tests
test _range    ... bench:     33329 ns/iter (+/- 417)
test assembly  ... bench:     33299 ns/iter (+/- 58)
test enumerate ... bench:     33318 ns/iter (+/- 83)
test iter      ... bench:     33311 ns/iter (+/- 130)
test position  ... bench:     33300 ns/iter (+/- 47)

test result: ok. 0 passed; 0 failed; 0 ignored; 5 measured
````

Fixes #18193

src/libcollections/vec.rs

@@ -56,6 +56,7 @@ use core::cmp::{Ordering};
 use core::default::Default;
 use core::fmt;
 use core::hash::{self, Hash};
+use core::intrinsics::assume;
 use core::iter::{repeat, FromIterator, IntoIterator};
 use core::marker::{self, ContravariantLifetime, InvariantType};
 use core::mem;
@@ -1587,8 +1588,12 @@ impl<T> AsSlice<T> for Vec<T> {
     #[stable(feature = "rust1", since = "1.0.0")]
     fn as_slice(&self) -> &[T] {
         unsafe {
+            let p = *self.ptr;
+            if cfg!(not(stage0)) { // NOTE remove cfg after next snapshot
+                assume(p != 0 as *mut T);
+            }
             mem::transmute(RawSlice {
-                data: *self.ptr,
+                data: p,
                 len: self.len
             })
         }