[4.0] Fix dominator tree update in array specialization

[aschwaighofer]

We need to walk the dominator tree starting at the header and update the
dominator of all nodes outside the loop we hit i.e the dominator tree nodes that
are immediately dominated 'by the loop' instead of only updating dominated exit
blocks.

• Explanation: The dominator tree update in array specialization pass is broken in cases where there are blocks outside the loop that are dominated by the loop but are not exit blocks. As a result the dominator tree was not correctly updated and we would detect this and crash in the compiler when run with -sil-verify-all. We used to only update exit blocks that are dominated by the loop header. The fix is to walk the dominator tree starting from the header and update all blocks that are immediately dominated by the loop (i.e first blocks that we hit along the domtree walk that are not loop blocks).

• Scope of Issue: This bug has existed since the introduction of the array specialization pass.

• Risk: Low. Should only affect cases that were failing before

• Testing: Added swift regression test

rdar://34523864

[aschwaighofer]

@swift-ci Please test

[aschwaighofer]

@swift-ci Please Test Source Compatibility

[aschwaighofer]

@swift-ci Please Test Source Compatibility

[eeckstein]

LGTM!