NIR: Fix some issues related to value range tracking
After bug #111308 was submitted, I came up with a couple theories for the cause. Most of them were not the cause of that bug, but I have been able to create piglit tests (piglit!110 (merged)) that exhibit the failures. This series fixes the piglit tests and the Unigine Heaven rendering.
The last commit also adds range tracking for nir_op_fpow. Somehow I missed that on the first go. The test for pow added in the piglit MR still passes with this addition.