[AMDGPU] Make SGPR spills exec mask agnostic

Explicitly set the exec mask for SGPR spills and reloads.
This fixes a bug where SGPR spills to memory could be incorrect
if the exec mask was 0 (or differed between spill and reload).

Additionally pack scalar subregisters (upto 16/32 per VGPR),
so that the majority of scalar types can be spilt or reloaded
with a simple memory access.  This should amortize some of the
additional overhead of manipulating the exec mask.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D80282