[InstCombine] Handle GEP scalar/vector base mismatch (PR55363)

30a12f3f switched the type check
to use the GEP result type rather than the GEP operand type.
However, the GEP result types may match even if the operand types
don't, in case GEPs with scalar/vector base and vector index
are compared.

Fixes https://github.com/llvm/llvm-project/issues/55363.