[SCEV] Make MulOpsInlineThreshold lower to avoid excessive compilation time

MulOpsInlineThreshold option of SCEV is defaulted to 1000, which is inadequately high.
When constructing SCEVs of expressions like:

  x1 = a * a
  x2 = x1 * x1
  x3 = x2 * x2
    ...

We actually have huge SCEVs with max allowed amount of operands inlined.
Such expressions are easy to get from unrolling of loops looking like

  x = a
  for (i = 0; i < n; i++)
    x = x * x

Or more tricky cases where big powers are involved. If some non-linear analysis
tries to work with a SCEV that has 1000 operands, it may lead to excessively long
compilation. The attached test does not pass within 1 minute with default threshold.

This patch decreases its default value to 32, which looks much more reasonable if we
use analyzes with complexity O(N^2) or O(N^3) working with SCEV.

Differential Revision: https://reviews.llvm.org/D34397

llvm-svn: 305882