[X86][AVX] Attempt to fold a scaled index into a gather/scatter scale immediate (PR13310)

If the index operand for a gather/scatter intrinsic is being scaled (self-addition or a shl-by-immediate) then we may be able to fold that scaling into the intrinsic scale immediate value instead.

Fixes PR13310.

Differential Revision: https://reviews.llvm.org/D108539