[x86] instcombine more cases of insertps into a shufflevector

This is a follow-on to D8833 (insertps optimization when the zero mask is not used).

In this patch, we check for the case where the zmask is used, but both input vectors
to the insertps intrinsic are the same operand or the zmask overrides the destination
lane. This lets us replace the 2nd shuffle input operand with the zero vector.

Differential Revision: http://reviews.llvm.org/D9257

llvm-svn: 235810