[GISel][NFC]: Add methods to speed up insertion into GISelWorklist

https://reviews.llvm.org/D58073

Speed up insertion during the initial populating phase into the
GISelWorkList by deferring repeatedly resizing the DenseMap.
This results in ~10% improvement in the combiner passes, and
~3% speedup in the Legalizer.

reviewed by: aemerson.

llvm-svn: 354093