[X86] Remove zext from vXi32 to vXi64 on indices of gather/scatter...

[X86] Remove zext from vXi32 to vXi64 on indices of gather/scatter instructions if we can prove the pre-extended value is positive.

Gather/scatter can implicitly sign extend from i32->i64 on indices. So if we know the sign bit of the input to a zext is 0 we can use the implicit extension.

llvm-svn: 321209