[llvm] Add PreLegalizer pattern matching for degrees GL extension (PR #216506)

[Kaitlin Peng via llvm-commits]

================
@@ -34,10 +34,16 @@ def matrix_multiply_lowering
                         [{ return Helper.matchMatrixMultiply(*${root}); }]),
                     (apply [{ Helper.applyMatrixMultiply(*${root}); }])>;
 
+def degrees_lowering
----------------
kmpeng wrote:

nit: order-wise I feel like this should go up with the distance/faceforward matchers rather than the matrix ones.

https://github.com/llvm/llvm-project/pull/216506