ac: add unreachable() in ac_build_image_opcode()

ac: add unreachable() in ac_build_image_opcode()

To silent the following compiler warning:

common/ac_llvm_build.c: In function ‘ac_build_image_opcode’:
common/ac_llvm_build.c:1080:3: warning: ‘name’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   snprintf(intr_name, sizeof(intr_name), "%s%s%s%s.v4f32.%s.v8i32",
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    name,
    ~~~~~
    a->compare ? ".c" : "",
    ~~~~~~~~~~~~~~~~~~~~~~~
    a->bias ? ".b" :
    ~~~~~~~~~~~~~~~~
    a->lod ? ".l" :
    ~~~~~~~~~~~~~~~
    a->deriv ? ".d" :
    ~~~~~~~~~~~~~~~~~
    a->level_zero ? ".lz" : "",
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    a->offset ? ".o" : "",
    ~~~~~~~~~~~~~~~~~~~~~~
    type);
    ~~~~~

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>