x86/MPX: fix address size handling

x86/MPX: fix address size handling

While address overrides are ignored in 64-bit mode (and hence shouldn't
really result in an error, but upon v1 converting this to a warning I
was told otherwise), trying to use 16-bit addressing is documented to
result in #UD, and hence the assembler should reject the attempt. (The
added test case at once also checks that bndc{l,n,u} won't accept
16-bit register operands.)