[RISCV] Support negative constants in CompressInstEmitter

Summary:
Some compressed instructions match against negative values; store
immediates as a signed value such that these patterns will now match
the intended instructions.

Reviewers: asb, lenary, PaoloS

Reviewed By: asb

Subscribers: rbar, johnrusso, sabuasal, niosHD, kito-cheng, shiva0217, jrtc27, MaskRay, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, PkmX, jocewei, psnobl, benna, Jim, s.egerton, pzheng, sameer.abuasal, apazos, luismarques, evandro, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76767