Explicitly truncate/widen shift value in OptimizeVisitor::PowExp

[ibuclaw]

In certain scenarios, a ^^ expression may be wider or narrower than the default tshiftcnt type (int).

Example:

foreach(i; 1..table.length - 1)
  immutable strideLength = 2 ^^ i;

As I've brought up in previous PRs, this kind of type punning in the front-end IR doesn't sit too well with strict type-checked backend, so adding in an explicit cast fixes tree verification ICE's I'm seeing in gdc-5.0.

[ibuclaw]

In case it isn't obvious, this area optimises 2UL ^^ i into 1UL << cast(int)(i * 1UL).