Here is the output when GOSSAFUNC is set:

The GT case:

 v100 = LoadReg <float64> v7 : F3
 v117 = FMOVDconst <float64> [1.7976931348623157e+308] : F4
 v81 = FCMPU <flags> v100 v117
 v17 = FGreaterThan <bool> v81 : R3
 v83 = CMPWconst <flags> [0] v17
 NE v83 -> b32 b12

The NE case:

v18 = FCMPU <flags> v100 v100
NE v18 -> b21 b17 (unlikely)

Found these lines commented out in the PPC64.rules so I changed them:

-// (NE (CMPWconst [0](FLessThan cc)) yes no) -> (FLT cc yes no)
-// (NE (CMPWconst [0](FLessEqual cc)) yes no) -> (FLE cc yes no)
-// (NE (CMPWconst [0](FGreaterThan cc)) yes no) -> (FGT cc yes no)
-// (NE (CMPWconst [0](FGreaterEqual cc)) yes no) -> (FGE cc yes no)
+(NE (CMPWconst [0](FLessThan cc)) yes no) -> (FLT cc yes no)
+(NE (CMPWconst [0](FLessEqual cc)) yes no) -> (FLE cc yes no)
+(NE (CMPWconst [0](FGreaterThan cc)) yes no) -> (FGT cc yes no)
+(NE (CMPWconst [0](FGreaterEqual cc)) yes no) -> (FGE cc yes no)

That seemed to fix the case I show above but in this function I see some of this:

b15: <- b13
v18 = FCMPU v7 v7
v29 = NotEqual v18 DEAD
v20 = CMPWconst [0] v29 DEAD
NE v18 -> b21 b17
b17: <- b15
v88 = FCMPU v8 v8
v33 = NotEqual v88 DEAD
v14 = CMPWconst [0] v33 DEAD
NE v88 -> b21 b3

v18 05850 (/home/boger/golang/base/go/src/math/mod.go:24) FCMPU F3, F3
b15 05851 (/home/boger/golang/base/go/src/math/mod.go:24) BNE 5906
v88 05852 (/home/boger/golang/base/go/src/math/mod.go:24) FCMPU F1, F1
b17 05853 (/home/boger/golang/base/go/src/math/mod.go:24) BNE 5906

We need to be sure that's an actual optimizer glitch, and not code that is checking that either operand is a NaN. And guess what:

    if y == 0 || IsInf(x, 0) || IsNaN(x) || IsNaN(y) {
        return NaN()
    }

I assigned myself the bug, if you want to do the uncommenting CL that works too.

CL https://golang.org/cl/31612 mentions this issue.