[RISCV] Collect Statistics on Compressed Instructions

Summary:
It is useful to keep statistics on how many instructions we have
compressed, so we can see if future changes are increasing or decreasing this
number.

Reviewers: asb, luismarques

Reviewed By: asb, luismarques

Subscribers: xbolva00, sameer.abuasal, hiraditya, rbar, johnrusso, simoncook, apazos, sabuasal, niosHD, kito-cheng, shiva0217, jrtc27, MaskRay, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, PkmX, jocewei, psnobl, benna, Jim, s.egerton, pzheng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67495