Fix PR6497, a bug where we'd fold a load into an addc

node which has a flag.  That flag in turn was used by an
already-selected adde which turned into an ADC32ri8 which
used a selected load which was chained to the load we
folded.  This flag use caused us to form a cycle.  Fix
this by not ignoring chains in IsLegalToFold even in
cases where the isel thinks it can.

llvm-svn: 97791