[MIPS] Add static_assert that all Fixups are handled in getFixupKind

[arichardson]

Summary

I recently added a new Fixup kind to our fork of LLVM but forgot to add
it to the table in MipsAsmBackend.cpp. With this static_assert the error
would have been caught instead of zero-initializing the array entries for
the new fixups.

[atanasyan]

LGTM

lib/Target/Mips/MCTargetDesc/MipsAsmBackend.cpp

 //
 //===----------------------------------------------------------------------===//
 //
 
 #include "MCTargetDesc/MipsAsmBackend.h"
 #include "MCTargetDesc/MipsFixupKinds.h"
 #include "MCTargetDesc/MipsMCExpr.h"
 #include "MCTargetDesc/MipsMCTargetDesc.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/MC/MCAsmBackend.h"
 #include "llvm/MC/MCAssembler.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCDirectives.h"
 #include "llvm/MC/MCELFObjectWriter.h"
 #include "llvm/MC/MCFixupKindInfo.h"
 #include "llvm/MC/MCObjectWriter.h"
 #include "llvm/MC/MCSubtargetInfo.h"
Optional<MCFixupKind> MipsAsmBackend::getFixupKind(StringRef Name) const {
   return StringSwitch<Optional<MCFixupKind>>(Name)
       .Case("R_MIPS_NONE", (MCFixupKind)Mips::fixup_Mips_NONE)
       .Case("R_MIPS_32", FK_Data_4)
       .Default(MCAsmBackend::getFixupKind(Name));
 }
 
 const MCFixupKindInfo &MipsAsmBackend::
 getFixupKindInfo(MCFixupKind Kind) const {
-  const static MCFixupKindInfo LittleEndianInfos[Mips::NumTargetFixupKinds] = {
+  const static MCFixupKindInfo LittleEndianInfos[] = {
     // This table *must* be in same the order of fixup_* kinds in
     // MipsFixupKinds.h.
     //
     // name                    offset  bits  flags
     { "fixup_Mips_NONE",         0,      0,   0 },
     { "fixup_Mips_16",           0,     16,   0 },
     { "fixup_Mips_32",           0,     32,   0 },
     { "fixup_Mips_REL32",        0,     32,   0 },
const static MCFixupKindInfo LittleEndianInfos[] = {
     { "fixup_MICROMIPS_TLS_DTPREL_HI16", 0,     16,   0 },
     { "fixup_MICROMIPS_TLS_DTPREL_LO16", 0,     16,   0 },
     { "fixup_MICROMIPS_GOTTPREL",        0,     16,   0 },
     { "fixup_MICROMIPS_TLS_TPREL_HI16",  0,     16,   0 },
     { "fixup_MICROMIPS_TLS_TPREL_LO16",  0,     16,   0 },
     { "fixup_Mips_SUB",                  0,     64,   0 },
     { "fixup_MICROMIPS_SUB",             0,     64,   0 }
   };
+  static_assert(array_lengthof(LittleEndianInfos) == Mips::NumTargetFixupKinds,
+                "Not all MIPS little endian fixup kinds added!");
 
-  const static MCFixupKindInfo BigEndianInfos[Mips::NumTargetFixupKinds] = {
+  const static MCFixupKindInfo BigEndianInfos[] = {
     // This table *must* be in same the order of fixup_* kinds in
     // MipsFixupKinds.h.
     //
     // name                    offset  bits  flags
     { "fixup_Mips_NONE",         0,      0,   0 },
     { "fixup_Mips_16",          16,     16,   0 },
     { "fixup_Mips_32",           0,     32,   0 },
     { "fixup_Mips_REL32",        0,     32,   0 },
const static MCFixupKindInfo BigEndianInfos[] = {
     { "fixup_MICROMIPS_TLS_DTPREL_HI16", 16,     16,   0 },
     { "fixup_MICROMIPS_TLS_DTPREL_LO16", 16,     16,   0 },
     { "fixup_MICROMIPS_GOTTPREL",        16,     16,   0 },
     { "fixup_MICROMIPS_TLS_TPREL_HI16",  16,     16,   0 },
     { "fixup_MICROMIPS_TLS_TPREL_LO16",  16,     16,   0 },
     { "fixup_Mips_SUB",                   0,     64,   0 },
     { "fixup_MICROMIPS_SUB",              0,     64,   0 }
   };
+  static_assert(array_lengthof(BigEndianInfos) == Mips::NumTargetFixupKinds,
+                "Not all MIPS big endian fixup kinds added!");
 
   if (Kind < FirstTargetFixupKind)
     return MCAsmBackend::getFixupKindInfo(Kind);
 
   assert(unsigned(Kind - FirstTargetFixupKind) < getNumFixupKinds() &&
           "Invalid kind!");
 
   if (IsLittle)