nir/algebraic: Change the default cursor location when replacing a unary op

nir/algebraic: Change the default cursor location when replacing a unary op

If the expression tree that is being replaced has a unary operation at
its root, set the cursor (location where new instructions are inserted)
at the source instruction instead.

This doesn't do much now because there are very few patterns that have a
unary operation as the root.  Almost all of the patterns that do have a
unary operation as the root have inot.  All of the shaders that are
affected by this commit have expression trees with an inot at the root.

This change prevents some significant, spurious caused by the next
commit.  There is further explanation in the large comment added in
the code.

I also considered a couple other options that may still be worth exploring.

1. Add some mark-up to the search pattern to denote where new
   instructions should be added.  I considered using "@" to denote the
   cursor location.  For example,

    (('fneg', ('fadd@', a, b)), ...)

2. To prevent other kinds of unintended code motion, add the ability to
   name expressions in the search pattern so that they can be reused in
   the replacement.  For example,

   (('bcsel', ('ige', ('find_lsb=b', a), 0), ('find_lsb', a), -1), b),

   An alternative would be to add some kind of CSE at the time of
   inserting the replacements.  Create a new instruction, then check to
   see if it already exists.  That option might be better overall.

Over the years I know Matt has heard me complain, "I added a pattern
that just deleted an instruction, but it added a bunch of spills!"  This
was always in large, complex shaders that are very hard to analyze.  I
always blamed these cases on the scheduler being dumb.  I am now very
suspicious that unintended code motion was the real problem.

All Gen4+ Intel platforms had similar results. (Tiger Lake shown)
total instructions in shared programs: 17611405 -> 17611333 (<.01%)
instructions in affected programs: 18613 -> 18541 (-0.39%)
helped: 41
HURT: 13
helped stats (abs) min: 1 max: 18 x̄: 4.46 x̃: 4
helped stats (rel) min: 0.27% max: 5.68% x̄: 1.29% x̃: 1.34%
HURT stats (abs)   min: 1 max: 20 x̄: 8.54 x̃: 7
HURT stats (rel)   min: 0.30% max: 4.20% x̄: 2.15% x̃: 2.38%
95% mean confidence interval for instructions value: -3.29 0.63
95% mean confidence interval for instructions %-change: -0.95% 0.02%
Inconclusive result (value mean confidence interval includes 0).

total cycles in shared programs: 338366118 -> 338365223 (<.01%)
cycles in affected programs: 257889 -> 256994 (-0.35%)
helped: 42
HURT: 15
helped stats (abs) min: 2 max: 120 x̄: 39.38 x̃: 34
helped stats (rel) min: 0.04% max: 2.55% x̄: 0.86% x̃: 0.76%
HURT stats (abs)   min: 6 max: 204 x̄: 50.60 x̃: 34
HURT stats (rel)   min: 0.11% max: 4.75% x̄: 1.12% x̃: 0.56%
95% mean confidence interval for cycles value: -30.39 -1.02
95% mean confidence interval for cycles %-change: -0.66% -0.02%
Cycles are helped.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1359>