[AMDGPU] Handle SI_MASKED_UNREACHABLE in instruction emitter

SI_MASKED_UNREACHABLE does not have machine instruction encoding.
It needs special handling in AMDGPUAsmPrinter::EmitInstruction like some
other pseudo instructions.

This patch fixes compilation failure of RadeonRays.

Differential Revision: https://reviews.llvm.org/D32364

llvm-svn: 301025