[SYCL] Add barrier before leader guard in LowerWGScope pass

[againull]

Currently barrier is added only to merge basic block. But barrier must
be added before leader guard too. As an example, let's consider the
following pseudo code which is generated by LowerWGScope:

1  __local int *done;
2  kernel test() {
3   int done_wi;
4   int count = 0;
5   do {
6     count++;
7     if (get_local_linear_id() == 0)
8       *done = (count == 2);
9     barrier(CLK_LOCAL_MEM_FENCE);
10    done_wi = *done;
11  } while (!done_wi);
12 }

Step 1. All WIs execute stmt 3 - stmt 9. This is the first time all WIs
encounter the barrier.
Step 2. WI0 execute stmt 10 - stmt 11 and this time done is false, and
then execute stmt 5 - stmt 9, it set done to true. This is the second
time WI0 encounter the barrier.
Step 3. Other WIs begin executing stmt 10 - stmt 11 they will see done
is true so they can't reach barrier now.

To resolve this problem barrier must be added before leader guard:

...
barrier(CLK_LOCAL_MEM_FENCE);
if (get_local_linear_id() == 0)
...

Signed-off-by: Artur Gainullin [email protected]

[kbobrovs]

NIT: would be nice to add a comment why the extra barrier is needed.

[kbobrovs]

NIT: would be nice to add the C++ source of the SYCL kernel used to generate this IR as a comment.