[InstSimplify] Pass pointer and indices separately to SimplifyGEPInst.

This doesn't require callers to put the pointer operand and the indices
in a container like a vector when calling the function. This is not
really an issue with the existing callers. But when using it from
IRBuilder the inputs are available as separate pointer value and indices
ArrayRef.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D117038