[dev.ssa] cmd/compile: allocate the flag register in a separate pass

[dev.ssa] cmd/compile: allocate the flag register in a separate pass

Spilling/restoring flag values is a pain to do during regalloc.
Instead, allocate the flag register in a separate pass.  Regalloc then
operates normally on any flag recomputation instructions.

Change-Id: Ia1c3d9e6eff678861193093c0b48a00f90e4156b
Reviewed-on: https://go-review.googlesource.com/17694
Reviewed-by: David Chase <drchase@google.com>