[test] Put aix-xcoff-huge-relocs.ll under expensive checks

It is an order of magnitude slower than the second slowest test
according to obj/llvm/test/.lit_test_times.txt.

The two slowest are:
 2.870437e+02 CodeGen/PowerPC/aix-xcoff-huge-relocs.ll
 2.850697e+01 tools/llvm-readobj/ELF/file-header-machine-types.test

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D102190