target/i386: validate VEX.W for AVX instructions

Instructions in VEX exception class 6 generally look at the value of
VEX.W.  Note that the manual places some instructions incorrectly in
class 4, for example VPERMQ which has no non-VEX encoding and no legacy
SSE analogue.  AMD does a mess of its own, as documented in the comment
that this patch adds.

Most of them are checked for VEX.W=0, and are listed in the manual
(though with an omission) in table 2-16; VPERMQ and VPERMPD check for
VEX.W=1, which is only listed in the instruction description.  Others,
such as VPSRLV, VPSLLV and the FMA3 instructions, use VEX.W to switch
between a 32-bit and 64-bit operation.

Fix more of the class 4/class 6 mismatches, and implement the check for
VEX.W in TCG.

Acked-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>