[dev.ssa] cmd/compile/internal/gc: reduce genValue redundancy

[dev.ssa] cmd/compile/internal/gc: reduce genValue redundancy

Add an asm field to opcodeTable containing the Prog's as field.
Then instructions that fill the Prog the same way can be collapsed
into a single switch case.

I'm still thinking of a better way to reduce redundancy, but
I think this might be a good temporary solution to prevent duplication
from getting out of control. What do you think?

Change-Id: I0c4a0992741f908bd357ee2707edb82e76e4ce61
Reviewed-on: https://go-review.googlesource.com/11130
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>