instead of migrating it to the MC instruction encoder, just

rip out the implementation of X86InstrInfo::GetInstSizeInBytes.
The code being ripped out just implemented a copy and hacked up
version of the (old) instruction encoder, and is buggy and 
terrible in other ways.  Since "GetInstSizeInBytes" is really 
only there to support the JIT's "NeedsExactSize" hook (which
noone is using), just rip out the code.  I will rip out the
NeedsExactSize hook next.

This resolves rdar://7617809 - switch X86InstrInfo::GetInstSizeInBytes to use X86MCCodeEmitter

llvm-svn: 109149