[CodeGen] Properly propagating Calling Convention information when lowering vector arguments

When joining the legal parts of vector arguments into its original value
during the lower of Formal Arguments in SelectionDAGBuilder, the Calling
Convention information was not being propagated for the handling of each
individual parts. The same did not happen when lowering calls, causing a
mismatch.

This patch fixes the issue by properly propagating the Calling
Convention details.

This fixes Bugzilla #47001.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D86715