[llvm-mc-fuzzer] Support untested instruction discovery for variable length...

[llvm-mc-fuzzer] Support untested instruction discovery for variable length instruction sets like microMIPS.

Summary:
For fixed length instructions, we can use -max_len to limit the fuzzer to a
single instruction. This doesn't work for variable length instruction sets
since a 4-byte input could consist of one 4-byte instruction or two 2-byte
instructions.

This patch adds a --insn-limit to llvm-mc-fuzzer to limit the input in
terms of instructions processed.

Reviewers: kcc

Subscribers: kcc, llvm-commits

Differential Revision: http://reviews.llvm.org/D12960

llvm-svn: 248253