[llvm-exegesis] Fix PR39021.

Summary:
The `set` statements was incorrectly reading the value of the local variable and
setting the value of the parent variable.

Reviewers: tycho, gchatelet, john.brawn

Subscribers: mgorny, tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D52343

llvm-svn: 342865