Disassembly of instructions can fail when llvm-objdump is not given the right set of
architecture features, for example when the source is compiled with:
clang -march=..+ext1+ext2
and disassembly is attempted with:
llvm-objdump -mattr=+ext1
This patch avoids further analysing unknown instructions (as was happening
before) when disassembly has failed.
Differential Revision: https://reviews.llvm.org/D73531
