[dev.ssa] cmd/compile/ssa: add comparison ops

[dev.ssa] cmd/compile/ssa: add comparison ops

Increase SSA coverage of functions in the
standard library from 20.79% to 27.81%.

The most significant unimplemented items are now:

 10.16%  2597 SSA unimplemented: zero for type error not implemented
  8.44%  2157 SSA unimplemented: addr: bad op DOTPTR
  7.98%  2039 SSA unimplemented: unhandled OLITERAL 7
  6.29%  1607 SSA unimplemented: unhandled expr OROR
  4.73%  1209 SSA unimplemented: unhandled expr LEN
  4.55%  1163 SSA unimplemented: unhandled expr LROT
  3.42%   874 SSA unimplemented: unhandled OLITERAL 6
  2.46%   629 SSA unimplemented: unhandled expr DOT
  2.41%   615 SSA unimplemented: zero for type []byte not implemented
  2.02%   516 SSA unimplemented: unhandled stmt CALLMETH
  1.90%   486 SSA unimplemented: unhandled expr ANDAND
  1.79%   458 SSA unimplemented: unhandled expr CALLINTER
  1.69%   433 SSA unimplemented: unhandled stmt SWITCH
  1.67%   428 SSA unimplemented: unhandled expr CALLMETH
  1.67%   426 SSA unimplemented: unhandled expr CLOSUREVAR

Change-Id: I40959b22993c4f70784b4eca472cae752347879c
Reviewed-on: https://go-review.googlesource.com/11452
Reviewed-by: Keith Randall <khr@golang.org>