[InstCombine][X86][SSE] Replace sign/zero extension intrinsics with native IR

Now that we are generating sane codegen for vector sext/zext nodes on SSE targets, this patch uses instcombine to replace the SSE41/AVX2 pmovsx and pmovzx intrinsics with the equivalent native IR code.

Differential Revision: http://reviews.llvm.org/D11503

llvm-svn: 243303