[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>

src/cmd/compile/internal/ssa/regalloc.go

@@ -985,7 +985,7 @@ func (s *regAllocState) regalloc(f *Func) {
 			args = append(args[:0], v.Args...)
 			for _, i := range regspec.inputs {
 				mask := i.regs
-				if mask == flagRegMask {
+				if mask == f.Config.flagRegMask {
 					// TODO: remove flag input from regspec.inputs.
 					continue
 				}