cmd/compile: get MIPS64 SSA working

- implement *, /, %, shifts, Zero, Move.
- fix mistakes in comparison.
- fix floating point rounding.
- handle RetJmp in assembler (which was not handled, as a consequence
  Duff's device was disabled in the old backend.)

all.bash now passes with SSA on.

Updates #16359.

Change-Id: Ia14eed0ed1176b5d800592080c8f53dded7fe73f
Reviewed-on: https://go-review.googlesource.com/27592
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -471,7 +471,7 @@ func (s *regAllocState) init(f *Func) {
 	}
 
 	// Figure out which registers we're allowed to use.
-	s.allocatable = s.f.Config.gpRegMask | s.f.Config.fpRegMask
+	s.allocatable = s.f.Config.gpRegMask | s.f.Config.fpRegMask | s.f.Config.specialRegMask
 	s.allocatable &^= 1 << s.SPReg
 	s.allocatable &^= 1 << s.SBReg
 	if s.f.Config.hasGReg {
@@ -1302,7 +1302,7 @@ func (s *regAllocState) regalloc(f *Func) {
 			// We assume that a control input can be passed in any
 			// type-compatible register. If this turns out not to be true,
 			// we'll need to introduce a regspec for a block's control value.
-			s.allocValToReg(v, s.compatRegs(v.Type), false, b.Line)
+			b.Control = s.allocValToReg(v, s.compatRegs(v.Type), false, b.Line)
 			// Remove this use from the uses list.
 			vi := &s.values[v.ID]
 			u := vi.uses