x86: ignore high register select bit(s) in 32- and 16-bit modes

x86: ignore high register select bit(s) in 32- and 16-bit modes

While commits 9889cbb14e ("Check invalid mask registers") and
abfcb414b9 ("X86: Ignore REX_B bit for 32-bit XOP instructions") went a
bit into the right direction, this wasn't quite enough:
- VEX.vvvv has its high bit ignored
- EVEX.vvvv has its high bit ignored together with EVEX.v'
- the high bits of {,E}VEX.vvvv should not be prematurely zapped, to
  allow proper checking of them when the fields has to hold al ones
- when the high bits of an immediate specify a register, bit 7 is
  ignored