[MCA][NFCI] Minor changes to InstrBuilder and Instruction.

This is based on the assumption that most simulated instructions don't define
more than one or two registers. This is true for example on x86, where
most instruction definitions don't declare more than one register write.

The default code region size has been increased from 8 to 16. This is based on
the assumption that, for small microbenchmarks, the typical code snippet size is
often less than 16 instructions.

mca::Instruction now uses bitfields to pack flags.
No functional change intended.