[X86] Strengthen the setting of inline asm constraints for fp regclasses.

This is similar to r268953, but for floating point and vector register
classes.

Explanations:
The setting of the inline asm constraints was implicitly relying on the
order of the register classes in the file generated by tablegen.
Since, we do not have any control on that order, make sure we do not
depend on it anymore.

llvm-svn: 268973