Add to opcode_mix analyzer: branch density; memory density; arithmetic intensity

Some common metrics for workload characterization are based purely on opcodes and would fit in as additions to the existing opcode_mix analyzer:

+ branch density
+ memory density: we have loads+stores and total instrs from the basic_counts tool but this fits in with the other density metrics here
+ arithmetic intensity: FLOPS vs memops; integer too? DR's API identifies floating point sub-types (wondering whether this is implemented for aarch64; it is there for x86): https://github.com/DynamoRIO/dynamorio/blob/master/core/ir/instr_api.h#L1845