Revert "glsl: Fix constant-folding for reciprocal expressions"

Revert "glsl: Fix constant-folding for reciprocal expressions"

This reverts commit b3cf92aa916ee0537ee37723c23a9897ac9cd3e0.

The reverted commit prevented constant-folding of reciprocal expressions
when the reciprocated expression was 0. However, since the spec allows
division by zero, constant-folding *is* permissible in this case.

From Section 5.9 of the GLSL 1.20 spec:
    Dividing by zero does not cause an exception but does result in an
    unspecified value.