Revert "Support constant folding of exponential built-ins"
Revert "Support constant folding of exponential built-ins"

Part of a chain breaking the clang build:

http://build.chromium.org/p/chromium.gpu.fyi/builders/GPU%20Linux%20Builder/builds/33588

This reverts commit 1c89caef47a27c63a7d0f988e74816692a9b9b39.

Change-Id: I36e080d8f64d4d4ca5de04bebbd119db83956d0e
Reviewed-on: https://chromium-review.googlesource.com/265185
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Tested-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

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