gallivm: Use a minimax polynomial for exp2 in range [0,1] instead [-0.5,5].

gallivm: Use a minimax polynomial for exp2 in range [0,1] instead [-0.5,5].

The advantage of range[-0.5, 0.5] is that it doesn't require floor (for
which intrinsics are only available in SSE4.1).

But the EXP opcode pretty much forces us to use floor, and there is a
good floor approximation around truncation available anyway.

This fixes EXP failures in VShader DCT.