[X86][SSE] Vector integer to float conversion memory folding

Added missing memory folding for the (V)CVTDQ2PS instructions - we can safely fold these (but not the (V)CVTDQ2PD versions which have a register/memory size discrepancy in the source operand). I've added a test case demonstrating that stack folding now works.

Differential Revision: http://reviews.llvm.org/D5981