Avoid shuffle self-assignment in EXPENSIVE_CHECKS builds

Some versions of libstdc++ perform self-assignment in std::shuffle. This
breaks the EXPENSIVE_CHECKS builds of TableGen due to an incorrect assertion
in libstdc++.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85828.

Fixes https://llvm.org/PR37652

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D98167