tccgen.c: Avoid undefined behaviour in constant propagation.
tccgen.c: Avoid undefined behaviour in constant propagation.

run4flat/tinycc,avih/tinycc,seyko2/tinycc,run4flat/tinycc,seyko2/tinycc,avih/tinycc,andreiw/tinycc,mirror/tinycc,metaeducation/tcc,seyko2/tinycc,metaeducation/tcc,mingodad/tinycc,seyko2/tinycc,run4flat/tinycc,metaeducation/tcc,mingodad/tinycc,mirror/tinycc,andrewchambers/tccmirror,code4game/tinycc,mirror/tinycc,andrewchambers/tccmirror,mingodad/tinycc,code4game/tinycc,metaeducation/tcc,mirror/tinycc,andreiw/tinycc,code4game/tinycc,andrewchambers/tccmirror,andrewchambers/tccmirror,seyko2/tinycc,metaeducation/tcc,andreiw/tinycc,run4flat/tinycc,avih/tinycc,run4flat/tinycc,mingodad/tinycc,avih/tinycc,andrewchambers/tccmirror