[GlobalISel] Extract handleAssignments out of AArch64CallLowering

This function seems target-independent so far: all the target-specific behaviour
is isolated in the CCAssignFn and the ValueHandler (which we're also extracting
into the generic CallLowering).

The intention is to use this in the ARM backend.

Differential Revision: https://reviews.llvm.org/D27045

llvm-svn: 288658