[AMDGPU] Implement whole wave register spill

To reduce the register pressure during allocation,
when the allocator spills a virtual register that
corresponds to a whole wave mode operation, the
spill loads and restores should be activated for
all lanes by temporarily flipping all bits in exec
register to one just before the spills. It is not
implemented in the compiler as of today and this
patch enables the necessary support.

This is a pre-patch before the SGPR spill to virtual
VGPR lanes that would eventually causes the whole
wave register spills during allocation.

Reviewed By: arsenm, cdevadas

Differential Revision: https://reviews.llvm.org/D143759