[GlobalISel][Tablegen] Assign small opcodes to pseudos

Sort pseudo instructions first while emitting enum's for target
instructions info. That puts them close to each other and to generic
G_* opcodes for GlobalISel. This makes it easier to build small jump
tables over opcodes that could be directly embedded into MatchTable's
Tablegen'erated for GlobalISel's InstructionSelect.

Reviewed By: bogner

Differential Revision: https://reviews.llvm.org/D47240

llvm-svn: 333135