glsl: When lowering non-constant array indexing, respect existing conditions

glsl: When lowering non-constant array indexing, respect existing conditions

If the non-constant index was in the LHS of an assignment, any
existing condititon on that assignment would be lost.

Fixes i965 piglit:

    fs-temp-array-mat[234]-col-row-wr
    fs-temp-array-mat[234]-index-col-row-wr
    fs-temp-array-mat[234]-index-col-wr
    fs-temp-array-mat[234]-index-row-wr
    vs-varying-array-mat[234]-index-col-wr

Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 5f83dfe5b70337bcffe215f7c32d0b862b5e7a3b)