[X86] Clean up some mir tests with INLINEASM to avoid regdef or to correct the...

[X86] Clean up some mir tests with INLINEASM to avoid regdef or to correct the immediate for the regdef.

The immediate used for the regdef is the encoding for the register
class in the enum generated by tablegen. This encoding will change
any time a new register class is added. Since the number is part
of the input, this means it can become stale.

This change modifies some test to avoid this kind of immediate
all together. And updates one test to use the current encoding of
GR64.