The cmd/compile/internal/ssa/gen directory can't depend on cmd/internal/gc
because that package doesn't exist in go1.4.  Use strings instead of
constants from that package.

The asm fields seem somewhat redundant to the opcode names we
conventionally use.  Maybe we can just trim the lowercase from the end
of the op name?  At least by default?

Change-Id: I96e8cda44833763951709e2721588fbd34580989
Reviewed-on: https://go-review.googlesource.com/11129Reviewed-by: Michael Matloob <michaelmatloob@gmail.com>