[x86] Rename In32BitMode predicate to Not64BitMode

That's what it actually means, and with 16-bit support it's going to be
a little more relevant since in a few corner cases we may actually want
to distinguish between 16-bit and 32-bit mode (for example the bare 'push'
aliases to pushw/pushl etc.)

Patch by David Woodhouse

llvm-svn: 197768