Make the slice iterator optimise better.

Due to the null pointer optimisation, LLVM was leaving totally useless
null pointer checks in tight loops using this iterator. We compile
`Some(a_reference)` to a no-op, and matching against None in an
`Option<&T>` is just comparing against 0, this means that the `None`
branch in `match Some(a_reference) { None => ... }` (as appears
effectively in a loop using `slice::Iter`) can't be eliminated unless
`a_reference` is known to be non-null.

LLVM currently doesn't seem to understand this in every situation (often
due to inlining, I am told), particularly painfully in ones involving
this iterator, but using `assume` means it understands almost always.

    for i in x {
        test::black_box(i);
    }

Before:

    .LBB0_4:
    	leaq	4(%rcx), %rsi
    	testq	%rcx, %rcx
    	je	.LBB0_5
    	movq	%rcx, (%rsp)
    	#APP
    	#NO_APP
    	cmpq	%rsi, %rax
    	movq	%rsi, %rcx
    	jne	.LBB0_4

After:

    .LBB0_4:
    	movq	%rcx, (%rsp)
    	addq	$4, %rcx
    	#APP
    	#NO_APP
    	cmpq	%rcx, %rax
    	jne	.LBB0_4

This leads to significantly better unrolling and vectorisation too.

Author: huonw

src/libcore/slice.rs

@@ -40,6 +40,7 @@ use cmp::{Ordering, PartialEq, PartialOrd, Eq, Ord};
 use cmp::Ordering::{Less, Equal, Greater};
 use cmp;
 use default::Default;
+use intrinsics;
 use iter::*;
 use marker::Copy;
 use num::Int;
@@ -690,6 +691,13 @@ macro_rules! iterator {
                             let old = self.ptr;
                             self.ptr = self.ptr.offset(1);
 
+                            // LLVM doesn't properly recognise that
+                            // the `&` is never null without this, and
+                            // so doesn't see that the Option we
+                            // create here is always Some, due to the
+                            // null-pointer optimisation.
+                            intrinsics::assume(!old.is_null());
+
                             Some(transmute(old))
                         }
                     }
@@ -723,6 +731,8 @@ macro_rules! iterator {
                         } else {
                             self.end = self.end.offset(-1);
 
+                            // see above
+                            intrinsics::assume(!self.end.is_null());
                             Some(transmute(self.end))
                         }
                     }