[RegBankSelect] Add a method to avoid splitting while repairing.

The previous choice of the insertion points for repairing was
straightfoward but may introduce some basic block or edge splitting. In
some situation this is something we can avoid.
For instance, when repairing a phi argument, instead of placing the
repairing on the related incoming edge, we may move it to the previous
block, before the terminators. This is only possible when the argument
is not defined by one of the terminator.

llvm-svn: 270232