Implement llvm-isel-fuzzer for fuzzing instruction selection
This implements a fuzzer tool for instruction selection, as described
in my [EuroLLVM 2017 talk].
The fuzzer must be given both libFuzzer args and llc-like args to
configure the backend. For example, to fuzz AArch64 GlobalISel at -O0,
you could invoke like so:
llvm-isel-fuzzer <corpus dirs> -ignore_remaining_args=1 \ -mtriple arm64-apple-ios -global-isel -O0
If you would like to seed the fuzzer with an initial corpus, simply
provide a directory of valid LLVM bitcode (not textual IR) as one of
the corpus dirs.