tgsi: simplify tgsi_shader_info::is_msaa_sampler checking

tgsi: simplify tgsi_shader_info::is_msaa_sampler checking

We assert that fullinst->Instruction.Texture != 0 above so no need to
check it in the conditional.  We also have the fullinst->Texture.Texture
value in a local variable, so use it.

Reviewed-by: José Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>