i965: Make Gen4-5 and Gen8+ ALT checks use ctx->_Shader too.

i965: Make Gen4-5 and Gen8+ ALT checks use ctx->_Shader too.

Commit c0347705 changed the Gen6-7 code to use ctx->_Shader rather than
ctx->Shader, but neglected to change the Gen4-5 or Gen8+ code.

This might fix SSO related bugs, but ALT mode is only used for ARB
programs, so if there's an actual problem, it's likely no one would
run into it.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>