Optimizing: Rewrite DCE's MarkReachableBlocks().

Replace a recursive implementation with a loop using a work
list to avoid stack overflow that we would presumably hit
for 702-LargeBranchOffset in host debug build with -O0, once
the DCE block elimination is enabled for methods containing
try-catch.

Bug: 24133462
Change-Id: I41288ba368722bcb5d68259c7c147552c8928099