x86: refine TPAUSE and UMWAIT

x86: refine TPAUSE and UMWAIT

Allowing 64-bit registers is misleading here: Elsewhere these get allowed
when there's no difference between either variant, because of 32-bit
destination registers having their upper halves zeroed in 64-bit mode.
Here, however, they're source registers, and hence specifying 64-bit
registers would lead to the ambiguity of whether the upper 32 bits
actually matter.

Additionally, for proper code generation in 16-bit mode, IgnoreSize is
needed on both.

And finally, just like for e.g. MONITOR/MWAIT, add variants with all
input registers explicitly specified.