[llvm-profgen] Use hot threshold for context merging and trimming

Without preinliner, we need to tune down the cold count cutoff to merge/trim more context to limit profile size for large components. However it doesn't make sense for cold threshold to be higher than hot threshold, so we now change to use hot threshold as merging/trimming cut off instead.

Differential Revision: https://reviews.llvm.org/D110212