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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@321209 91177308-0d34-0410-b5e6-96231b3b80d8