T-API: optimized index calculation in reduction operations

check_regression=*OCL*Norm*:*OCL*MinMax*:*OCL*Dot*:*OCL*Sum*:*OCL*CountNonZeroFix*
test_modules=core
test_filter=*OCL*Norm*:*OCL*MinMax*:*OCL*Dot*:*OCL*Sum*:*OCL*CountNonZeroFix*
build_examples=OFF