[X86][SSE41] Combine insertion of zero scalars into vector blends with zero

Part 1 of 2
This patch attempts to replace the insertion of zero scalars with a vector blend with zero, avoiding the use of the integer insertion instructions (which are particularly slow on many targets).
(Part 2 will add support for combining multiple blends-with-zero).

Differential Revision: http://reviews.llvm.org/D17483

llvm-svn: 261743