glsl: Avoid division-by-zero during constant-folding

glsl: Avoid division-by-zero during constant-folding

Avoid division-by-zero when constant-folding the following expression
types:
    ir_unop_rsq
    ir_binop_div
    ir_binop_mod

Fixes bugs:
https://bugs.freedesktop.org//show_bug.cgi?id=33306
https://bugs.freedesktop.org//show_bug.cgi?id=33508

Fixes Piglit tests:
glslparsertest/glsl2/div-by-zero-01.frag
glslparsertest/glsl2/div-by-zero-02.frag
glslparsertest/glsl2/div-by-zero-03.frag
glslparsertest/glsl2/modulus-zero-01.frag
glslparsertest/glsl2/modulus-zero-02.frag

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