[X86] Properly encode a 32-bit address with an index register and no base register...

[X86] Properly encode a 32-bit address with an index register and no base register in 16-bit mode.

In 16-bit mode we can encode a 32-bit address using 0x67 prefix.
We were failing to do this when the index register was a 32-bit
register, the base register was not present, and the displacement
fit in 16-bits.

Fixes PR46866.