Sink: Handle instruction sink when a user is dead

Summary:
  The current instruction sink pass uses findNearestCommonDominator of all users to find block to sink the instruction to.
However, a user may be in a dead block, which will result in unexpected behavior.

This patch handles such cases by skipping dead blocks. This patch fixes:
https://bugs.llvm.org/show_bug.cgi?id=47415

Reviewers:
  MaskRay, arsenm

Differential Revision:
  https://reviews.llvm.org/D89166