[mips][wip] Disable the selection of mixed microMIPS/MIPS code

[sdardis]

Summary

This patch disables the selection of microMIPS and MIPS instructions and
patterns that are defined in TableGen files. Certain pseudo instructions
are selectable in both modes, but these are expanded by C++ code.

WIP notes: There are still some test failures when this patch is applied.

[atanasyan]

LGTM

[atanasyan]

Equivalent changes have been applied by D51483 / rL341338.

lib/Target/Mips/MipsSubtarget.h

public:
   }
   // Hard float for mips16 means essentially to compile as soft float
   // but to use a runtime library for soft float that is written with
   // native mips32 floating point instructions (those runtime routines
   // run in mips32 hard float mode).
   bool inMips16HardFloat() const {
     return inMips16Mode() && InMips16HardFloat;
   }
-  bool inMicroMipsMode() const { return InMicroMipsMode; }
-  bool inMicroMips32r6Mode() const { return InMicroMipsMode && hasMips32r6(); }
+  bool inMicroMipsMode() const { return InMicroMipsMode && !InMips16Mode; }
+  bool inMicroMips32r6Mode() const {
+    return inMicroMipsMode() && hasMips32r6();
+  }
   bool hasDSP() const { return HasDSP; }
   bool hasDSPR2() const { return HasDSPR2; }
   bool hasDSPR3() const { return HasDSPR3; }
   bool hasMSA() const { return HasMSA; }
   bool disableMadd4() const { return DisableMadd4; }
   bool hasEVA() const { return HasEVA; }
   bool hasMT() const { return HasMT; }
   bool hasCRC() const { return HasCRC; }
   bool hasVirt() const { return HasVirt; }
   bool hasGINV() const { return HasGINV; }
   bool useIndirectJumpsHazard() const {
     return UseIndirectJumpsHazard && hasMips32r2();
   }
   bool useSmallSection() const { return UseSmallSection; }
 
-  bool hasStandardEncoding() const { return !inMips16Mode(); }
+  bool hasStandardEncoding() const {
+    return !inMips16Mode() && !inMicroMipsMode();
+  }
 
   bool useSoftFloat() const { return IsSoftFloat; }
 
   bool useLongCalls() const { return UseLongCalls; }
 
   bool enableLongBranchPass() const {
-    return hasStandardEncoding() || allowMixed16_32();
+    return hasStandardEncoding() || inMicroMipsMode() || allowMixed16_32();
   }
 
   /// Features related to the presence of specific instructions.
   bool hasExtractInsert() const { return !inMips16Mode() && hasMips32r2(); }
   bool hasMTHC1() const { return hasMips32r2(); }
 
   bool allowMixed16_32() const { return inMips16ModeDefault() |
                                         AllowMixed16_32; }