String repeat does not work with runtime values

[kabra1110]

from lpython import i32

def test_str_repeat():
    i: i32 = 10
    s: str = 'a'
    print(s * i)

test_str_repeat()

This gives a compiler error, occurring here

                right_int = ASR::down_cast<ASR::IntegerConstant_t>(
                                                   ASRUtils::expr_value(right))->m_n;

An assertion to check non-nullptr in down_cast fails.

[certik]

It's assuming that "i" is a compile time constant, but it's a variable in this case.

[kabra1110]

I did try changing the code to use Integer_t instead, but the issue appears to be it being a nullptr.