Implement optimizations for the strchr and llvm.memset library calls.

Neither of these activated as many times as was hoped:

strchr:
9 MultiSource/Applications/siod
1 MultiSource/Applications/d
2 MultiSource/Prolangs-C/archie-client
1 External/SPEC/CINT2000/176.gcc/176.gcc

llvm.memset:
no hits

llvm-svn: 21669