[dev.ssa] cmd/compile: handle boolean values for SSA on ARM

[dev.ssa] cmd/compile: handle boolean values for SSA on ARM

Fix hardcoded flag register mask in ssa/flagalloc.go by auto-generating
the mask.

Also fix a mistake (in previous CL) about conditional branches.

Progress on SSA backend for ARM. Still not complete. Now "container/ring"
package compiles and tests passed.

Updates #15365.

Change-Id: Id7c8805c30dbb8107baedb485ed0f71f59ed6ea8
Reviewed-on: https://go-review.googlesource.com/23093
Reviewed-by: Keith Randall <khr@golang.org>