[X86][MMX] Handle i32->mmx conversion using movd

[X86][MMX] Handle i32->mmx conversion using movd

Implement a BITCAST dag combine to transform i32->mmx conversion patterns
into a X86 specific node (MMX_MOVW2D) and guarantee that moves between
i32 and x86mmx are better handled, i.e., don't use store-load to do the
conversion..

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228293 91177308-0d34-0410-b5e6-96231b3b80d8