target/i386: generalize operand size "ph" for use in CVTPS2PD

CVTPS2PD only loads a half-register for memory, like CVTPH2PS.  It can
reuse the "ph" packed half-precision size to load a half-register,
but rename it to "xh" because it is now a variation of "x" (it is not
used only for half-precision values).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>