[CodeGenPrepare] Avoid a scalable-vector crash in ctlz/cttz

This patch fixes a crash when despeculating ctlz/cttz intrinsics with
scalable-vector types. It is not safe to speculatively get the size of
the vector type in bits in case the vector type is not a fixed-length type. As
it happens this isn't required as vector types are skipped anyway.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D112141