Hello,
this optimization is a drop of callgrind instruction count by 43 mln
for ztrsub() – it had 89 mln instructions normally, now it has 46 mln:

ztrsub() callgrind I-count:
1> (89873326 - 46452836) / 1000000.0
43,4205

total callgrind I-count
2> (7906279976 - 7858975498) / 1000000.0
47,3045

 89,873,326  ztrsub [/usr/local/bin/zsh-5.6.2-dev-1-2noopt]
vs
 46,452,836  ztrsub [/usr/local/bin/zsh-5.6.2-dev-1-2yesopt]

Ran this callgrind probe twice to confirm. That said, `zsh -i -c exit'
nor a performance test script show any gain. I wonder why.. But
callgrind doesn't lie, and looking at the code one can understand why
I-count dropped by factor of 2. The patch has many context lines to
see whole function.
-- 
Sebastian Gniazdowski
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