x64Emitter: add different bitnesses of CVT*2SI
x64Emitter: add CVTTPD2DQ
Jit64: implement fctiw/fctiwz
Jit64: use BLENDVPD in ps_sel

As magumagu noted, BLENDVPD actually _is_ useful for ps_sel (but only in
combination with CMPPD).
Jit64: add fselx, based on the already done ps_sel
Jit64: use BLENDVPD in fsel