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The Follow pass in the assembler backend reorders and copies
instructions. This even applies to hand-written assembly code,
which in many cases don't want to be reordered. Now that the
SSA compiler does a good job for laying out instructions, the
benefit of this pass is very little:

AMD64: (old = with Follow, new = without Follow)
name                      old time/op    new time/op    delta
BinaryTree17-12              2.78s ± 1%     2.79s ± 1%  +0.44%  (p=0.000 n=20+19)
Fannkuch11-12                3.11s ± 0%     3.31s ± 1%  +6.16%  (p=0.000 n=19+19)
FmtFprintfEmpty-12          50.9ns ± 1%    51.6ns ± 3%  +1.40%  (p=0.000 n=17+20)
FmtFprintfString-12          127ns ± 0%     128ns ± 1%  +0.88%  (p=0.000 n=17+17)
FmtFprintfInt-12             122ns ± 0%     123ns ± 1%  +0.76%  (p=0.000 n=20+19)
FmtFprintfIntInt-12          185ns ± 1%     186ns ± 1%  +0.65%  (p=0.000 n=20+19)
FmtFprintfPrefixedInt-12     192ns ± 1%     202ns ± 1%  +4.99%  (p=0.000 n=20+19)...