Lazily allocate DenseMaps.
Lazily allocate DenseMaps.

This makes lookup slightly more expensive but it's worth it, unused
DenseMaps are common in LLVM code apparently.

1% speedup on clang -O3 bzip2.c
4% speedup on clang -O3 oggenc.c (Release build of clang on i386/linux)

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@127088 91177308-0d34-0410-b5e6-96231b3b80d8

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