[RISCV] Replace map with set in getReqFeatures

Summary:
Use a set in getReqFeatures() in RISCVCompressInstEmitter instead of a map
because the index we save is not needed.

This also fixes bug 41666.

Reviewers: llvm-commits, apazos, asb, nickdesaulniers

Reviewed By: asb

Subscribers: Jim, nickdesaulniers, rbar, johnrusso, simoncook, niosHD, kito-cheng, shiva0217, jrtc27, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, PkmX, jocewei, psnobl, benna

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61412

llvm-svn: 362968