bugpoint: make the number of trim iterations a compile-time constant

Around 10 year ago Chris limited this code to a single iteration by just
dropping a break into the loop body. We now make the number of trim iterations
a compile time constant to be able to play with it and see if this can
improve the bugpoint results. We currently use with '3' still a small and
conservative value, but this can be adjusted in the future, if needed.

I tried to look for a trivial test case, but did not succeed yet.

llvm-svn: 243247