Make use of the dereferenceable attribute instead of nonnull where we can.

[luqmana]

LLVM recently added a new attribute, dereferenceable: http://reviews.llvm.org/D4449

This patch adds a dereferencable attribute. In some sense, this is a companion to the nonnull attribute, but specifies that the pointer is known to be dereferencable in the same sense as a pointer generated by alloca is known to be dereferencable.

With rust, everywhere that we previously marked nonnull we can actually mark as dereferenceable (which implies nonnull) since we know the size. That is, except for one case: when generating calls for TyVisitor. It seems like we haven't substituted the self type (so we have ty_param) and just treat it as an opaque pointer so I just left that bit as nonnull.

With this, LLVM can for example hoist a load out of a loop where it previously couldn't:

pub fn baz(c: &uint, n: uint) -> uint {
    let mut res = 0;
    for i in range(0, n) {
        if i > 0 {
            res += *c * i;
        }
    }
    res
}

Before:

define i64 @baz(i64* noalias nocapture nonnull readonly, i64) unnamed_addr #0 {
entry-block:
  br label %for_loopback.outer

for_loopback.outer:                               ; preds = %then-block-33-, %entry-block
  %.ph = phi i64 [ %.lcssa, %then-block-33- ], [ 0, %entry-block ]
  %res.0.ph = phi i64 [ %8, %then-block-33- ], [ 0, %entry-block ]
  br label %for_loopback

for_exit:                                         ; preds = %for_loopback
  %res.0.ph.lcssa = phi i64 [ %res.0.ph, %for_loopback ]
  ret i64 %res.0.ph.lcssa

for_loopback:                                     ; preds = %for_loopback.outer, %for_body
  %2 = phi i64 [ %4, %for_body ], [ %.ph, %for_loopback.outer ]
  %3 = icmp ult i64 %2, %1
  br i1 %3, label %for_body, label %for_exit

for_body:                                         ; preds = %for_loopback
  %4 = add i64 %2, 1
  %5 = icmp eq i64 %2, 0
  br i1 %5, label %for_loopback, label %then-block-33-

then-block-33-:                                   ; preds = %for_body
  %.lcssa = phi i64 [ %4, %for_body ]
  %.lcssa15 = phi i64 [ %2, %for_body ]
  %6 = load i64* %0, align 8                     ; <------- this load
  %7 = mul i64 %6, %.lcssa15
  %8 = add i64 %7, %res.0.ph
  br label %for_loopback.outer
}

After:

define i64 @baz(i64* noalias nocapture readonly dereferenceable(8), i64) unnamed_addr #0 {
entry-block:
  %2 = load i64* %0, align 8                    ; <------- load once instead
  br label %for_loopback.outer

for_loopback.outer:                               ; preds = %then-block-33-, %entry-block
  %.ph = phi i64 [ %.lcssa, %then-block-33- ], [ 0, %entry-block ]
  %res.0.ph = phi i64 [ %8, %then-block-33- ], [ 0, %entry-block ]
  br label %for_loopback

for_exit:                                         ; preds = %for_loopback
  %res.0.ph.lcssa = phi i64 [ %res.0.ph, %for_loopback ]
  ret i64 %res.0.ph.lcssa

for_loopback:                                     ; preds = %for_loopback.outer, %for_body
  %3 = phi i64 [ %5, %for_body ], [ %.ph, %for_loopback.outer ]
  %4 = icmp ult i64 %3, %1
  br i1 %4, label %for_body, label %for_exit

for_body:                                         ; preds = %for_loopback
  %5 = add i64 %3, 1
  %6 = icmp eq i64 %3, 0
  br i1 %6, label %for_loopback, label %then-block-33-

then-block-33-:                                   ; preds = %for_body
  %.lcssa = phi i64 [ %5, %for_body ]
  %.lcssa15 = phi i64 [ %3, %for_body ]
  %7 = mul i64 %2, %.lcssa15
  %8 = add i64 %7, %res.0.ph
  br label %for_loopback.outer
}