[X86][SSE] Sign extension for target vector sizes less than 128 bits (pt1)

This patch adds support for sign extension for sub 128-bit vectors, such as to v2i32. It concatenates with UNDEF subvectors up to 128-bits, performs the sign extension (i.e. as v4i32) and then extracts the target subvector.

Patch 1/2 of D10589 - the second patch covers the conversion of v2i8/v2i16 to v2f64.

llvm-svn: 241323