[llvm-exegesis] Make duplicate snippet repetitor produce whole snippets (#77224)

Currently, the duplicate snippet repetitor will truncate snippets that
do not exactly divide the minimum number of instructions. This patch
corrects that behavior by making the duplicate snippet repetitor
duplicate the snippet in its entirety until the minimum number of
instructions has been reached.

This makes the behavior consistent with the loop snippet repetitor,
which will execute at least `--num-repetitions` (soon to be renamed
`--min-instructions`) instructions.