[InstCombine] make icmp-mul fold more efficient

There's probably a lot more like this (see also comments in D33338 about responsibility), 
but I suspect we don't usually get a visible manifestation.

Given the recent interest in improving InstCombine efficiency, another potential micro-opt
that could be repeated several times in this function: morph the existing icmp pred/operands
instead of creating a new instruction.

llvm-svn: 303860