dx: Deprecate SsaBasicBlock.isReachable()

Changes SsaMethod.computeReachability() to return the reachability
bitmap is calculates and remove unnecessary isReachable field from
SsaBasicBlock.

Bug: 65231722
Test: dalvik/dx/tests/run-all-tests
Change-Id: Ife3073b00243b0a7ae3d43845d2241f610520023