[IRBuilder] Remove positivity check from CreateAlignmentAssumption()

Summary:
An alignment should be non-zero positive power-of-two, anything and everything else is UB.
We should not have that check for all these prerequisites here, it's just UB.
Also, that was likely confusing middle-end passes.

While there, `CreateIntCast()` should be called with `/*isSigned*/ false`.
Think about it, there are two explanations: "An alignment should be positive",
therefore the sign bit is unset, so `zext` and `sext` is equivalent.
Or a second one: you have `i2 0b10` - a valid alignment,
now you `sext` it: `i2 0b110` - no longer valid alignment.

Reviewers: craig.topper, jyknight, hfinkel, erichkeane, rjmccall

Reviewed By: hfinkel, rjmccall

Subscribers: hfinkel, llvm-commits

Differential Revision: https://reviews.llvm.org/D54653

llvm-svn: 352089