[X86] Don't accept (%si,%bp) 16-bit address expressions.

The second register is the index register and should only be %si or %di if used with a base register. And in that case the base register should be %bp or %bx.

This makes us compatible with gas.

We do still need to support both orders with Intel syntax which uses [bp+si] and [si+bp]

llvm-svn: 335384