cmd/compile: ignore g register in liveness analysis

In rare circumstances that we don't yet fully understand, the g
register can be spilled to the stack and then reloaded. If this
happens, liveness analysis sees a pointer load into a
non-general-purpose register and panics.

We should fix the root cause of this, but fix the build for now by
ignoring pointer loads into the g register.

For #25504.

Change-Id: I0dfee1af9750c8e9157c7637280cdf07118ef2ca
Reviewed-on: https://go-review.googlesource.com/114081
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/plive.go

@@ -461,6 +461,17 @@ func (lv *Liveness) regEffects(v *ssa.Value) (uevar, kill liveRegMask) {
 		for _, reg := range regs[:nreg] {
 			if reg.GCNum() == -1 {
 				if ptrOnly {
+					if reg.String() == "g" {
+						// Issue #25504: Sometimes we
+						// spill and reload the g
+						// register, which this sees
+						// as a pointer load into the
+						// g register. The g register
+						// isn't a GP register and
+						// can't appear in register
+						// maps. Ignore it.
+						continue
+					}
 					v.Fatalf("pointer in non-pointer register %v", reg)
 				} else {
 					continue