[X86][SSE] Improve recognition of i64 sitofp conversions that can be performed as i32 (PR29078)

Until AVX512DQ we only support i64/vXi64 sitofp conversion as scalars.

This patch sees if the sign bit extends far enough that we can truncate to a i32 type and then perform sitofp without loss of precision.

Differential Revision: https://reviews.llvm.org/D24345

llvm-svn: 281502