glsl: Fix constant-folding for reciprocal expressions

glsl: Fix constant-folding for reciprocal expressions

Do not constant-fold a reciprocal if any component of the reciprocated
expression is 0. For example, do not constant-fold `1 / vec4(0, 1, 2, 3)`.

Incorrect, previous behavior
----------------------------
Reciprocals were constant-folded even when some component of the
reciprocated expression was 0. The incorrectly applied arithmetic was:
   1 / 0 := 0
For example,
   1 / vec4(0, 1, 2, 3) = vec4(0, 1, 1/2, 1/3)

NOTE: This is a candidate for the 7.9 and 7.10 branches.