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

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