[GlobalISel] Allow multiple VRegs in ArgInfo. NFC

Allow CallLowering::ArgInfo to contain more than one virtual register.
This is useful when passes split aggregates into several virtual
registers, but need to also provide information about the original type
to the call lowering. Used in follow-up patches.

Differential Revision: https://reviews.llvm.org/D63548

llvm-svn: 364509