[NFC][TTI] Explicit use of VectorType

The API for shuffles and reductions uses generic Type parameters,
instead of VectorType, and so assertions and casts are used a lot.
This patch makes those types explicit, which means that the clients
can't be lazy, but results in less ambiguity, and that can only be a
good thing.

Bugzilla: https://bugs.llvm.org/show_bug.cgi?id=45562

Differential Revision: https://reviews.llvm.org/D78357