This change introduces two optimizations together,
one for recursive and one for non-recursive stacks.

For recursive stacks, we introduce the new entry
at the beginning of the cache, so it can be found first.
This adds an extra read and write.
While we're here, switch from fastrandn, which does a multiply,
to fastrand % n, which does a shift.

For non-recursive stacks, split the cache from [16]pcvalueCacheEnt
into [2][8]pcvalueCacheEnt, and add a very cheap associative lookup.

name                old time/op  new time/op  delta
StackCopyPtr-8       118ms ± 1%   106ms ± 2%  -9.56%  (p=0.000 n=17+18)
StackCopy-8         95.8ms ± 1%  87.0ms ± 3%  -9.11%  (p=0.000 n=19+20)
StackCopyNoCache-8   135ms ± 2%   139ms ± 1%  +3.06%  (p=0.000 n=19+18)

During make.bash, the association function used has this return distribution:

percent count  return value
 53.23% 678797 1
 46.74% 596094 0

It is definitely not perfect, but it is pretty good,
and that's all we need.

Change-Id: I2cabb1d26b99c5111bc28f427016a2a5e6c620fd
Reviewed-on: https://go-review.googlesource.com/c/110564
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>