[X86] Remove call to getZeroVector from materializeVectorConstant. Add isel...

[X86] Remove call to getZeroVector from materializeVectorConstant. Add isel patterns for zero vectors with all types.

The change to avx512-vec-cmp.ll is a regression, but should be
easy to fix. It occurs because the getZeroVector call was
canonicalizing both sides to the same node, then SimplifySelect
was able to simplify it. But since only called getZeroVector
on some VTs this isn't a robust way to combine this.

The change to vector-shuffle-combining-ssse3.ll is more
instructions, but removes a constant pool load so its unclear
if its a regression or not.

llvm-svn: 371350