[X86][MMX] Match MMX fp_to_sint conversions from XMM registers

We currently perform the various fp_to_sint XMM conversion and then transfer to the MMX register (on 32-bit via the stack).

This patch improves support for MOVDQ2Q XMM to MMX transfers and adds the XMM->MMX fp_to_sint direct conversion patterns. The SSE2 specifications are the same as for XMM->XMM and XMM->MMX rounding/exceptions/etc.

Differential Revision: https://reviews.llvm.org/D30868

llvm-svn: 298943