[BOLT] Prevent unsetting unknown control flow for split jump table

In case of a function with unknown control flow but with a single jump
table and a single jump table site, we attempt to match the jump table
and a site and update block successors using jump table targets.
Restrict this behavior for split jump tables which have targets in a
fragment function.

Fixes https://github.com/llvm/llvm-project/issues/60795.

Reviewed By: #bolt, rafauler

Differential Revision: https://reviews.llvm.org/D144602