[RISCV] Fix assert message operator

Summary:
Specifying assert message with an || operator makes the compiler interpret it
 as a bool. Changed it to &&.

Reviewers: asb, apazos

Reviewed By: asb

Subscribers: rbar, johnrusso, simoncook, jordy.potman.lists, sabuasal, niosHD, kito-cheng, shiva0217, zzheng, llvm-commits

Differential Revision: https://reviews.llvm.org/D45660

llvm-svn: 330148