[X86] Make Feature64Bit useful

We now only add +64bit to the CPU string for "generic" CPU. All other CPU names are assumed to have the feature flag already set if they support 64-bit. I've remove the implies from CMPXCHG8 so that Feature64Bit only comes in via CPUs or user passing -mattr=+64bit.

I've changed the assert to a report_fatal_error so it's not lost in Release builds.

The test updates are to fix things that tripped the new error.

Differential Revision: https://reviews.llvm.org/D51231

llvm-svn: 341022