gallivm: fix pow(0, y)
This passes piglit and vulkan cts
Fixes: dEQP-VK.glsl.builtin.precision.pow.highp* piglit arb_shader_precision pow tests
Edited by Dave Airlie
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This passes piglit and vulkan cts
Fixes: dEQP-VK.glsl.builtin.precision.pow.highp* piglit arb_shader_precision pow tests