Model ashr(shl(x, n), m) as mul(x, 2^(n-m)) when n > m

Given below case:

%y = shl %x, n %z = ashr %y, m

when n = m, SCEV models it as sext(trunc(x)). This patch tries to handle

the case where n > m by using sext(mul(trunc(x), 2^(n-m)))) as the SCEV

expression.