Revert "[mlir] Lower math dialect later in gpu-lower-to-nvvm-pipeline (#78556)"

This reverts commit 74bf0b1c. The test
always fails.

 | mlir/test/Dialect/GPU/test-nvvm-pipeline.mlir:23:16: error: CHECK-PTX: expected string not found in input
 |  // CHECK-PTX: __nv_expf

https://lab.llvm.org/buildbot/#/builders/61/builds/53789