Make smaller changes to compilers

[RustanLeino]

This PR contains some minor changes to the compilers. These changes have been carved off from a larger set of changes (for supporting general newtypes and is expressions), to simplify the reviewing of the latter.

The changes in this PR are:

• Rename parameter of EmitDowncastVariableAssignment (collectionElementType to sourceType).
• Add nativeType parameter to EmitBitvectorTruncation (to allow it to be different than bvType.NativeType).
• In some places, don't assume that native type of bitvector type is always as wide as the bitvector. (This happens to be true today, but it may not be true in the future (i) when a newtype can have a bitvector base type, which will happen soon, and (ii) if IntBoundedPool starts supporting bitvector types.

How has this been tested?

Since the changes are meant to preserve current behaviors (but with an eye to the up-and-coming PRs with larger changes), this PR does not make any changes to tests.

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.