[SCCP] Have the logic for replacing insts with constant in a single place.

The code was pretty much copy-pasted between SCCP and IPSCCP. The situation
became clearly worse after I introduced the support for folding structs in
SCCP.  This commit is NFC as we currently (still) skip the replacement
step in IPSCCP, but I'll change this soon.

llvm-svn: 275339