[GlobalISel] Accept multiple vregs for lowerCall's result
Change the interface of CallLowering::lowerCall to accept several
virtual registers for the call result, instead of just one. This is a
follow-up to D46018.
With this change, we no longer pack the virtual registers generated for
aggregates into one big lump before delegating to the target. Therefore,
the target can decide itself whether it wants to handle them as separate
pieces or use one big register.
ARM and AArch64 have been updated to use the passed in virtual registers
directly, which means we no longer need to generate so many
NFCI for AMDGPU, Mips and X86.
Differential Revision: https://reviews.llvm.org/D63550