Initialize SIInsertWaits::ExpInstrTypesSeen in the pass constructor.

This value may be used uninitialized in SIInsertWaits::insertWait.
Found with MemorySanitizer.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187869 91177308-0d34-0410-b5e6-96231b3b80d8