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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298631 91177308-0d34-0410-b5e6-96231b3b80d8