[X86][SSE] Replace VPMOVSX and (V)PMOVZX integer extension intrinsics with generic IR (clang)

The VPMOVSX and (V)PMOVZX sign/zero extension intrinsics can be safely represented as generic __builtin_convertvector calls instead of x86 intrinsics.

This patch removes the clang builtins and their use in the sse2/avx headers - a companion patch will remove/auto-upgrade the llvm intrinsics.

Note: We already did this for SSE41 PMOVSX sometime ago.

Differential Revision: http://reviews.llvm.org/D20684

llvm-svn: 271106