- FP_EXTEND only support extending from vectors with matching elements.
  This results in the scalarization of extending to v2f64 from v2f32,
  which will be legalized to v4f32 not matching with v2f64.
- add X86-specific VFPEXT supproting extending from v4f32 to v2f64.
- add BUILD_VECTOR lowering helper to recover back the original
  extending from v4f32 to v2f64.
- test case is enhanced to include different vector width.

llvm-svn: 161894