Implement *even more* factoring. In particular, if all of the instruction

strings starts out with a constant string, we emit the string first, using
a table lookup (instead of a switch statement).

Because this is usually the opcode portion of the asm string, the differences
between the instructions have now been greatly reduced.  This allows many
more case statements to be grouped together.

This patch also allows instruction cases to be grouped together when the
instruction patterns are exactly identical (common after the opcode string
has been ripped off), and when the differing operand is a MachineInstr
operand that needs to be formatted.

The end result of this is a mean and lean generated AsmPrinter!

llvm-svn: 19759