[X86][SSE] Reapplied: Improve vector ZERO_EXTEND by combining to ZERO_EXTEND_VECTOR_INREG

Generalise the existing SIGN_EXTEND to SIGN_EXTEND_VECTOR_INREG combine to support zero extension as well and get rid of a lot of unnecessary ANY_EXTEND + mask patterns.

Reapplied with a fix for PR26870 (avoid premature use of TargetConstant in ZERO_EXTEND_VECTOR_INREG expansion).

Differential Revision: http://reviews.llvm.org/D17691

llvm-svn: 263159