[RISCV] Update MC compression tests

Summary:
This patch updates MC tests related to compression in RISCV to
insure they work correctly with automatic compression and relaxation
enabled. This is the first part of a series of patches to implement
automatic compression for RISCV.

Reviewers: asb, apazos

Reviewed By: asb

Subscribers: shiva0217, efriedma, llvm-commits, asb, rbar, johnrusso, simoncook, jordy.potman.lists, apazos, niosHD, kito-cheng

Differential Revision: https://reviews.llvm.org/D43328

llvm-svn: 329441