Fix call to core_lib:is_simple/1
Fix call to core_lib:is_simple/1

core_lib:is_simple/1 isn't in R13B03, but it is in R13B02.
lfe_codegen:is_simple/1 is the version from R13B02.

rvirding/lfe,yurrriq/lfe,bsmr-erlang/lfe,jjhoo/lfe,jjl/lfe,lfe/lfe,jjl/lfe,yurrriq/lfe,bsmr-erlang/lfe,rvirding/lfe,jjhoo/lfe,arpunk/lfe,lfe/lfe,arpunk/lfe,oubiwann/lfe,oubiwann/lfe