Handle catch phis in code sinking.

When the user of an instruction we want to sink is a catch
phi, we should not look at predecessors (which don't map 1-1
for catch phis), but can only look at its dominator.

bug:37247890
Test: 647-sinking-catch
Change-Id: Ib64bd6f95d3ef45c394137e76819fa8d7d3d960a