[CSSPGO][llvm-profgen] Continue disassembling after illegal instruction is seen.

Previously we errored out when disassembling illegal instructions and there would be no profile generated. In fact illegal instructions are not uncommon and we'd better skip them and print "unknown" instead of erroring out. This matches the behavior of llvm-objdump (see disassembleObject in llvm-objdump.cpp).

Reviewed By: wlei, wenlei

Differential Revision: https://reviews.llvm.org/D97776