glsl: Apply the transformation "1/rsq(x) == sqrt(x)" in opt_algebraic.

glsl: Apply the transformation "1/rsq(x) == sqrt(x)" in opt_algebraic.

The comment was stale, because the lowering in question wasn't happening
in lower_instructions.cpp.  Presumably if the lowering ever moves there,
we can plumb the lowering mask through to opt_algebraic.

total instructions in shared programs: 1618696 -> 1616810 (-0.12%)
instructions in affected programs:     243018 -> 241132 (-0.78%)
GAINED:                                0
LOST:                                  0

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>