Conditional parameter evaluation

# Current:
@parameter
if nchunks <= 4:
    @parameter
    for chunk in range(nchunks):
        var idx = ...
        dest.store(idx, src.load[width=32](idx))
else:
    for chunk in range(nchunks):
        var idx = ...
        dest.store(idx, src.load[width=32](idx))

# Ideal:
@parameter(nchunks <= 4)
for chunk in range(nchunks):
    var idx = ...
    dest.store(idx, src.load[width=32](idx))