draw/llvm: improve debugging output a bit

draw/llvm: improve debugging output a bit

it's useful to know what the llvmbuildstore arguments are going to
be before executing it because it can crash and make sure to
print out the inputs only if we're not generating a gs because
it fetches inputs differently.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>