AsmWriterEmitter: increase the number of bits for OpcodeInfo from 32-bit to

48-bit if necessary, in order to reduce the generated code size.

We have 900 cases not covered by OpcodeInfo in ATT AsmWriter and more in Intel
AsmWriter and ARM AsmWriter.

This patch reduced the clang Release build size by 50k, running on a Mac Pro.

llvm-svn: 163814