[dev.ssa] cmd/compile: add register spec to getg

This omission was causing the new regalloc to fail.

Change-Id: If7ba7be38a436dbd0dd443828ddd7ebf6e35be0e
Reviewed-on: https://go-review.googlesource.com/13632Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/compile/internal/ssa/gen/AMD64Ops.go

@@ -290,7 +290,7 @@ func init() {
 
 		// Pseudo-ops
 		{name: "LoweredPanicNilCheck"},
-		{name: "LoweredGetG"},
+		{name: "LoweredGetG", reg: gp01},
 	}
 
 	var AMD64blocks = []blockData{

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

@@ -2123,7 +2123,11 @@ var opcodeTable = [...]opInfo{
 	},
 	{
 		name: "LoweredGetG",
-		reg:  regInfo{},
+		reg: regInfo{
+			outputs: []regMask{
+				65519, // .AX .CX .DX .BX .BP .SI .DI .R8 .R9 .R10 .R11 .R12 .R13 .R14 .R15
+			},
+		},
 	},
 
 	{