x86: correct VCVT{,U}SI2SD rounding mode handling

x86: correct VCVT{,U}SI2SD rounding mode handling

With EVEX.W clear the instruction doesn't ignore the rounding mode, but
(like for other insns without rounding semantics) EVEX.b set causes #UD.
Hence the handling of EVEX.W needs to be done when processing
evex_rounding_64_mode, not at the decode stages.

Derive a new 64-bit testcase from the 32-bit one to cover the different
EVEX.W treatment in both cases.