MIPS: Remove r6 DCHECKs for NEG.fmt instruction

These DCHECKs were causing several test failures or r6. They should not
be here because only NEG.PS format was removed in r6, NEG.S and NEG.D
instructions remain.

BUG=

Review-Url: https://codereview.chromium.org/2276563006
Cr-Commit-Position: refs/heads/master@{#38944}

src/mips/assembler-mips.cc

@@ -2492,13 +2492,11 @@ void Assembler::mov_s(FPURegister fd, FPURegister fs) {
 
 
 void Assembler::neg_s(FPURegister fd, FPURegister fs) {
-  DCHECK(!IsMipsArchVariant(kMips32r6));
   GenInstrRegister(COP1, S, f0, fs, fd, NEG_S);
 }
 
 
 void Assembler::neg_d(FPURegister fd, FPURegister fs) {
-  DCHECK(!IsMipsArchVariant(kMips32r6));
   GenInstrRegister(COP1, D, f0, fs, fd, NEG_D);
 }
 

src/mips64/assembler-mips64.cc

@@ -2818,13 +2818,11 @@ void Assembler::mov_s(FPURegister fd, FPURegister fs) {
 
 
 void Assembler::neg_s(FPURegister fd, FPURegister fs) {
-  DCHECK(kArchVariant == kMips64r2);
   GenInstrRegister(COP1, S, f0, fs, fd, NEG_D);
 }
 
 
 void Assembler::neg_d(FPURegister fd, FPURegister fs) {
-  DCHECK(kArchVariant == kMips64r2);
   GenInstrRegister(COP1, D, f0, fs, fd, NEG_D);
 }