MIPS: Remove madd.d usage for r6.

src/mips/assembler-mips.cc

Index: src/mips/assembler-mips.cc
diff --git a/src/mips/assembler-mips.cc b/src/mips/assembler-mips.cc
index f1e5dfb670c958e25b907696356596d2f90d2d17..d6a4f793f4d88f0f370f34c8e40792a68c84a80d 100644
--- a/src/mips/assembler-mips.cc
+++ b/src/mips/assembler-mips.cc
@@ -2065,6 +2065,7 @@ void Assembler::mul_d(FPURegister fd, FPURegister fs, FPURegister ft) {
 
 void Assembler::madd_d(FPURegister fd, FPURegister fr, FPURegister fs,
     FPURegister ft) {
+  DCHECK(IsMipsArchVariant(kMips32r2));
   GenInstrRegister(COP1X, fr, ft, fs, fd, MADD_D);
 }