fix two problems with machine sinking:
1. Sinking would crash when the first instruction of a block was
   sunk due to iterator problems.
2. Instructions could be sunk to their current block, causing an
   infinite loop.

This fixes PR3968


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@68787 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed