Background: X86 APX feature adds 16 registers within the same 64-bit
mode. PR #164638 is trying to extend such registers for FASTCC. However,
a blocker issue is calling convention cannot be changeable with or
without a feature.
The solution is to disable FASTCC if APX is not ready. This is an NFC
change to the final code generation, becasue X86 doesn't define an
alternative ABI for FASTCC in 64-bit mode. We can solve the potential
compatibility issue of #164638 with this patch.
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