[CSSPGO] Aggregation by the last K context frames for cold profiles

This change provides the option to merge and aggregate cold context by the last k frames instead of context-less name. By default K = 1 means the context-less one.

This is for better perf tuning. The more selective merging and trimming will rely on llvm-profgen's preinliner.

Reviewed By: wenlei, hoy

Differential Revision: https://reviews.llvm.org/D104131