[llvm-dev] Improve the performance of JamCRC

[Scott Smith via llvm-dev]

Lldb relies heavily on crc when loading shared libraries.  The existing
implementation is quite slow as it computes a byte at a time, creating a
long dependency chain.

Unfortunately the polynomial is not the same as the one implemented by x86
processors in SSE 4.2, but there's another way to make it faster by using
more lookup tables.

Zlib implements this, but rather than require zlib, I instead added the
relevant code to compute four bytes at a time in parallel.

A separate patch changes lldb to rely on JamCRC instead of its own
implementation.  This patch improves the performance, which brings my test
(starting lldb, breaking at main) from 47 seconds down to 36 seconds.
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