Prefetch the backreference hashtable bucket. #728

Job	Run time
Build and test cmake:gcc9	28s
Build and test cmake:clang15	33s
Build and test cmake:clang15:asan	50s
Build and test cmake:clang15:tsan	45s
Build and test cmake:clang15:ubsan	52s
Build and test cmake:qemu-arm-neon-gcc	1m 0s
Build and test cmake-osx:clang	47s
Build and test cmake-osx:gcc	38s
Build and test cmake-win64:msvc-rel	59s
Build and test cmake-win64:msvc-dbg	1m 1s
Build and test fuzz:clang15	29s
Build and test python3.10:clang15	1m 46s
Build and test python3.10-win	40s
Build and test maven	25s
Build and test bazel:root	33s
Build and test bazel:go	1m 3s
Build and test bazel:java	20s
Build and test bazel:research	20s
Build and test bazel-osx:root	1m 7s
Build and test bazel-osx:go	1m 58s
Build and test bazel-osx:java	23s
Build and test bazel-osx:research	1m 2s
Build and test bazel-win:root	59s
Build and test bazel-win:java	33s
Build and test bazel-win:research	2m 11s
	21m 42s

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