[llvm-exegesis] Do not try to assign random registers twice.

Doing a random assignment assigns both tested (forward) and back-to-back
(backward) instructions.

When none of the tested instruction and back-to-back instruction have
implicit aliasing, we're currently trying to do a random register
asignment twice.

Fix this (see PR26418).

Differential Revision: https://reviews.llvm.org/D90380