[X86][AVX2] Fix SIGN_EXTEND vector handling on AVX2 targets.

On AVX2 target we are poorly legalizing SIGN_EXTEND ops for which the input's legalized type doesn't have the same number of elements as the destination, resulting in an ANY_EXTEND followed by a SIGN_EXTEND_INREG.

This patch uses the existing SIGN_EXTEND -> SIGN_EXTEND_VECTOR_INREG combine to extend the input to the size of the result and using SIGN_EXTEND_VECTOR_INREG instead.

Differential Revision: http://reviews.llvm.org/D16994

llvm-svn: 260210