[llvm-link] fix IRMover returning wrong modified vector type

Modified scalable vector types weren't correctly returned at link-time.
The previous behaviour was a FixedVectorType was constructed
when expecting a ScalableVectorType. This commit has added a regression
test which re-creates the failure as well as a fix.

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D96953