[globalisel] Improve Legalizer debug output

[globalisel] Improve Legalizer debug output

* LegalizeAction should be printed by name rather than number
* Newly created instructions are incomplete at the point the observer first sees
  them. They are therefore recorded in a small vector and printed just before
  the legalizer moves on to another instruction. By this point, the instruction
  must be complete.

llvm-svn: 359481