[X86][MMX] Improve transfer from mmx to i32

Improve EXTRACT_VECTOR_ELT DAG combine to catch conversion patterns
between x86mmx and i32 with more layers of indirection.

Before:
  movq2dq %mm0, %xmm0
  movd %xmm0, %eax
After:
  movd %mm0, %eax

llvm-svn: 227969