[NFC] Simplify inner structure of InstructionPrecedenceTracking

Currently it has a set KnownBlocks that marks blocks as having cached
answers and a map FirstSpecialInsts that maps these blocks to first
special instructions in them. The value in the map is always non-null,
and for blocks that are known to have no special instructions the map
does not have an instance.

This patch removes KnownBlocks as obsolete. Instead, for blocks that
are known to have no special instructions, we just put a nullptr value.
This makes the code much easier to read.

llvm-svn: 341531