The execution mode of a kernel is stored in a global variable, whose value means:
- 0 - SPMD mode
- 1 - indicates generic mode
- 2 - SPMD mode execution with generic mode semantics
We are going to add support for SIMD execution mode. It will be come with another
execution mode, such as SIMD-generic mode. As a result, this value-based indicator
is not flexible.
This patch changes to bitset based solution to encode execution mode. Each
 - generic mode
 - SPMD mode
 - SIMD mode (will be added later)
In this way, 0x1 is generic mode, 0x2 is SPMD mode, and 0x3 is SPMD mode
execution with generic mode semantics. In the future after we add the support for
SIMD mode, 0b1xx will be in SIMD mode.