[CodeGen] Fix warning in getNode for EXTRACT_SUBVECTOR

[CodeGen] Fix warning in getNode for EXTRACT_SUBVECTOR

Fix a warning in getNode() when extracting a subvector from a
concat vector. We can simply replace the call to getVectorNumElements
with getVectorMinNumElements as this follows the defined behaviour
for EXTRACT_SUBVECTOR.

Differential Revision: https://reviews.llvm.org/D82746