[MIPS GlobalISel] Remove switch statement (fix r349346 for MSVC)

Temporarily remove switch statement without any case labels 
in function legalizeCustom in order to fix r349346 for MSVC.

llvm-svn: 349356

llvm/lib/Target/Mips/MipsLegalizerInfo.cpp

@@ -67,10 +67,5 @@ bool MipsLegalizerInfo::legalizeCustom(MachineInstr &MI,
 
   MIRBuilder.setInstr(MI);
 
-  switch (MI.getOpcode()) {
-  default:
-    return false;
-  }
-
-  return true;
+  return false;
 }