Hack around a GCC ICE that was fixed in GCC 6.2

Hack around a GCC ICE that was fixed in GCC 6.2

lib/Target/X86/AsmParser/X86AsmParser.cpp: In member function ‘void {anonymous}::X86AsmParser::SwitchMode(unsigned int)’:
lib/Target/X86/AsmParser/X86AsmParser.cpp:927:76:   in constexpr expansion of ‘AllModes.llvm::FeatureBitset::FeatureBitset(std::initializer_list<unsigned int>{((const unsigned int*)(& ._157)), 3u})’
include/llvm/MC/SubtargetFeature.h:56:12:   in constexpr expansion of ‘llvm::FeatureBitset::set(I)’
lib/Target/X86/AsmParser/X86AsmParser.cpp:927:76: internal compiler error: in fold_binary_loc, at fold-const.c:9921
     FeatureBitset AllModes({X86::Mode64Bit, X86::Mode32Bit, X86::Mode16Bit});
                                                                            ^

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369852 91177308-0d34-0410-b5e6-96231b3b80d8