draw_llvm: fix segfaults on non-SSE2 CPUs where it is disabled (v2)

draw_llvm: fix segfaults on non-SSE2 CPUs where it is disabled (v2)

Changes in v2:
- Change function name

Currently draw_llvm refuses to create itself on non-SSE2 CPUs due to
an alleged LLVM bug.

However, this is implemented improperly, because other parts of draw
still attempt to access draw->llvm, resulting in segfaults.

Instead, put the check in debug_get_option_draw_use_llvm, check that
before calling draw_llvm_create, and then check whether draw->llvm is
non-null everywhere else.