[llvm-exegesis] Replace --num-repetitions with --min-instructions (#77153)

This patch replaces --num-repetitions with --min-instructions to make it
more clear that the value refers to the minimum number of instructions
in the final assembled snippet rather than the number of repetitions of
the snippet. This patch also refactors some llvm-exegesis internal
variable names to reflect the name change.

Fixes #76890.