[globalisel][tablegen] Keep track of the insertion point while adding BuildMIAction's. NFC

Multi-instruction emission needs to ensure the the instructions are generated
a depth-first fashion. For example:
  (ADDWrr (SUBWrr a, b), c)
needs to emit the SUBWrr before the ADDWrr. However, our walk over
TreePatternNode's is highly context sensitive which makes it difficult to append
BuildMIActions in the order we want. To fix this, we now keep track of the
insertion point as we add actions. This will allow multi-insn emission to insert
BuildMI's in the correct place.

llvm-svn: 317029