Switch the fixed-length disassembler to be table-driven.

Refactor the TableGen'erated fixed length disassemblmer to use a
table-driven state machine rather than a massive set of nested
switch() statements.

As a result, the ARM Disassembler (ARMDisassembler.cpp) builds much more
quickly and generates a smaller end result. For a Release+Asserts build on
a 16GB 3.4GHz i7 iMac w/ SSD:

Time to compile at -O2 (averaged w/ hot caches):
  Previous: 35.5s
  New:       8.9s

TEXT size:
  Previous: 447,251
  New:      297,661

Builds in 25% of the time previously required and generates code 66% of
the size.

Execution time of the disassembler is only slightly slower (7% disassembling
10 million ARM instructions, 19.6s vs 21.0s). The new implementation has
not yet been tuned, however, so the performance should almost certainly
be recoverable should it become a concern.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161888 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCFixedLenDisassembler.h

@@ -0,0 +1,32 @@
+//===-- llvm/MC/MCFixedLenDisassembler.h - Decoder driver -------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// Fixed length disassembler decoder state machine driver.
+//===----------------------------------------------------------------------===//
+#ifndef MCFIXEDLENDISASSEMBLER_H
+#define MCFIXEDLENDISASSEMBLER_H
+
+namespace llvm {
+
+namespace MCD {
+// Disassembler state machine opcodes.
+enum DecoderOps {
+  OPC_ExtractField = 1, // OPC_ExtractField(uint8_t Start, uint8_t Len)
+  OPC_FilterValue,      // OPC_FilterValue(uleb128 Val, uint16_t NumToSkip)
+  OPC_CheckField,       // OPC_CheckField(uint8_t Start, uint8_t Len,
+                        //                uleb128 Val, uint16_t NumToSkip)
+  OPC_CheckPredicate,   // OPC_CheckPredicate(uleb128 PIdx, uint16_t NumToSkip)
+  OPC_Decode,           // OPC_Decode(uleb128 Opcode, uleb128 DIdx)
+  OPC_SoftFail,         // OPC_SoftFail(uleb128 PMask, uleb128 NMask)
+  OPC_Fail              // OPC_Fail()
+};
+
+} // namespace MCDecode
+} // namespace llvm
+
+#endif