[llvm] [NFC] Switch a number of DenseMaps to SmallDenseMaps for speedup (PR #109417)

[Jeremy Morse via llvm-commits]

jmorse wrote:

NB: I can't fix all the clang-format errors as the existing files aren't clean, and it'd transform this patch into a 75% clang-format patch.

https://github.com/llvm/llvm-project/pull/109417