[X86] Loosen memory folding requirements for cvtdq2pd and cvtps2pd instructions.

According to spec cvtdq2pd and cvtps2pd instructions don't require memory operand to be aligned
to 16 bytes. This patch removes this requirement from the memory folding table.

Differential Revision: https://reviews.llvm.org/D23919

llvm-svn: 280402