Rewrite the macro to be optimised better.

Code:

    cfor!{let (mut i, mut j) = (0u, 0u); i + j < x; { i += 1; j += 1 } {
        test::black_box(i);
    }}

Old IR:

    while_cond:                                       ; preds = %while_body, %entry-block
      %i.0 = phi i64 [ 0, %entry-block ], [ %i.1, %while_body ]
      %j.0 = phi i64 [ 0, %entry-block ], [ %j.1, %while_body ]
      %_first.0 = phi i1 [ false, %entry-block ], [ true, %while_body ]
      br i1 %_first.0, label %then-block-107-, label %next-block

    while_body:                                       ; preds = %next-block
      call void @llvm.lifetime.start(i64 8, i8* %1) #2
      store i64 %i.1, i64* %dummy.i, align 8
      call void asm "", "r,~{dirflag},~{fpsr},~{flags}"(i64* %dummy.i) #2
      call void @llvm.lifetime.end(i64 8, i8* %1) #2
      br label %while_cond

    then-block-107-:                                  ; preds = %while_cond
      %2 = add i64 %i.0, 1
      %3 = add i64 %j.0, 1
      br label %next-block

    next-block:                                       ; preds = %while_cond, %then-block-107-
      %i.1 = phi i64 [ %2, %then-block-107- ], [ %i.0, %while_cond ]
      %j.1 = phi i64 [ %3, %then-block-107- ], [ %j.0, %while_cond ]
      %4 = add i64 %j.1, %i.1
      %5 = icmp ult i64 %4, %0
      br i1 %5, label %while_body, label %while_exit

New IR:

    next-block:                                       ; preds = %next-block.lr.ph, %next-block
      %j.011 = phi i64 [ 0, %next-block.lr.ph ], [ %3, %next-block ]
      call void @llvm.lifetime.start(i64 8, i8* %2) #2
      store i64 %j.011, i64* %dummy.i, align 8
      call void asm "", "r,~{dirflag},~{fpsr},~{flags}"(i64* %dummy.i) #2
      call void @llvm.lifetime.end(i64 8, i8* %2) #2
      %3 = add i64 %j.011, 1
      %4 = shl i64 %3, 1
      %5 = icmp ult i64 %4, %0
      br i1 %5, label %next-block, label %while_exit.loopexit
Ignore optimisation files.
Squash the while_true lint warning with a hack.
Make the doc tests pass.
cargo does doc tests now.