[InstCombine] Improve inbounds preservation for ADD+GEP -> GEP+GEP (#135155)

Given that we have a "add nuw" and a "getelementptr inbounds nuw" like
this:
   %idx = add nuw i64 %idx1, %idx2
   %gep = getelementptr inbounds nuw i32, ptr %ptr, i64 %idx

Then we can preserve the "inbounds nuw" flag when transforming that into
two getelementptr instructions:
   %gep1 = getelementptr inbounds nuw i32, ptr %ptr, i64 %idx1
   %gep = getelementptr inbounds nuw i32, ptr %ptr, i64 %idx2

Similarly for just having "nuw", and "nusw nuw" instead of "inbounds nuw"
on the getelementptr.

Proof: https://alive2.llvm.org/ce/z/QSweWW