[X86] Move splat addends from the gather/scatter index operand to the base address

This can avoid a vector add and a constant pool load. Or an explicit broadcast in case of non-constant.

Also reverse the transform any time we encounter a constant index addend that can't be moved to base. In that case pull the constant from base into the index. This reduces code size needed for the displacement since we needed the index add anyway. Limit this to scale of 1 to avoid divisibility and wrap issues.

Authored by Craig.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D111595