switch the DomTreeNodes and IDoms maps in idom/postidom to a

switch the DomTreeNodes and IDoms maps in idom/postidom to a
DenseMap instead of an std::map.  This speeds up postdomtree
by about 25% and domtree by about 23%.  It also speeds up clients,
for example, domfrontier by 11%, mem2reg by 4% and ADCE by 6%.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@40826 91177308-0d34-0410-b5e6-96231b3b80d8