[X86] Don't turn avx masked.load with constant mask into masked.load+vselect...

[X86] Don't turn avx masked.load with constant mask into masked.load+vselect when passthru value is all zeroes.

This is intended to enable the use of an immediate blend or
more optimal instruction. But if the passthru is zero we don't
need any additional instructions.

llvm-svn: 362675