MIPS: Replaced unreachable code in DoStoreNamedField by an assertion.

Port r20194 (2dfda0f)

Original commit message:
This is part of the effort to assign an environment if and only if we can deopt.

Removed some related dead code on the way.

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/210023004

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20215 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mips/lithium-codegen-mips.cc

@@ -4036,6 +4036,9 @@ void LCodeGen::DoStoreNamedField(LStoreNamedField* instr) {
       instr->hydrogen()->value()->IsHeapObject()
           ? OMIT_SMI_CHECK : INLINE_SMI_CHECK;
 
+  ASSERT(!(representation.IsSmi() &&
+           instr->value()->IsConstantOperand() &&
+           !IsSmi(LConstantOperand::cast(instr->value()))));
   if (representation.IsHeapObject()) {
     Register value = ToRegister(instr->value());
     if (!instr->hydrogen()->value()->type().IsHeapObject()) {