Implement splitting of PHI nodes, allowing block extraction of BB's that have

PHI node entries from multiple outside-the-region blocks.  This also fixes
extraction of the entry block in a function.  Yaay.

This has successfully block extracted all (but one) block from the score_move
function in obsequi (out of 33).  Hrm, I wonder which block the bug is in.  :)

llvm-svn: 13489