cmd/compile: get MIPS64 SSA working

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>