isl_multi_*_get_at: extract out isl_multi_*_peek_at
isl_multi_*_get_at: extract out isl_multi_*_peek_at

This function will be reused in the next commit.

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl