[RISCV] Added isCompressibleInst() to estimate size in getInstSizeInBytes()

Summary:
Modified compression emitter tablegen backend to emit isCompressibleInst()
check which in turn is used by getInstSizeInBytes() to better estimate
instruction size. Note the generation of compressed instructions in RISC-V
happens late in the assembler therefore instruction size estimate might be off
if computed before.

Reviewers: lenary, asb, luismarques, lewis-revill

Reviewed By: asb

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68290

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