Revert "Use std::bitset for SubtargetFeatures"

This reverts commit r233055.

It still causes buildbot failures (gcc running out of memory on several platforms, and a self-host failure on arm), although less than the previous time.

llvm-svn: 233068
diff --git a/llvm/lib/Target/PowerPC/Disassembler/PPCDisassembler.cpp b/llvm/lib/Target/PowerPC/Disassembler/PPCDisassembler.cpp
index 605e74b..0ed0723 100644
--- a/llvm/lib/Target/PowerPC/Disassembler/PPCDisassembler.cpp
+++ b/llvm/lib/Target/PowerPC/Disassembler/PPCDisassembler.cpp
@@ -355,7 +355,7 @@
   uint32_t Inst =
       (Bytes[0] << 24) | (Bytes[1] << 16) | (Bytes[2] << 8) | (Bytes[3] << 0);
 
-  if (STI.getFeatureBits()[PPC::FeatureQPX]) {
+  if ((STI.getFeatureBits() & PPC::FeatureQPX) != 0) {
     DecodeStatus result =
       decodeInstruction(DecoderTableQPX32, MI, Inst, Address, this, STI);
     if (result != MCDisassembler::Fail)