Finally change the instruction looking map to be a densemap from

Finally change the instruction looking map to be a densemap from
record* -> instrinfo instead of std::string -> instrinfo.

This speeds up tblgen on cellcpu from 7.28 -> 5.98s with a debug
build (20%).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@98916 91177308-0d34-0410-b5e6-96231b3b80d8