Fix assertion in SmallDenseMap constructor with reserve from non-power-of-2 buckets count

`SmallDenseMap` constructor with reserve gets an arbitrary `NumInitBuckets` value and passes it below to `init` method.

If `NumInitBuckets` is greater then `InlineBuckets`, then `SmallDenseMap` initializes to large representation passing `NumInitBuckets` below to `DenseMap` initialization. `DenseMap::initEmpty` method asserts that initial buckets count must be a power of 2.

Proposed solution is to update `NumInitBuckets` value in `SmallDenseMap` constructor till the next power of 2. It should satisfy both `DenseMap` preconditions and required minimum buckets count for reservation.

Reviewed By: atrick

Differential Revision: https://reviews.llvm.org/D129825