[mlir][spirv] Fix math.ctlz for full zero bit cases

If the integer has all zero bits, GLSL FindUMsb would return -1.
So theoretically (31 - FindUMsb) should still give use the correct
result.  However, Adreno GPUshave issues with this:
https://buildkite.com/iree/iree-test-android/builds/6482#01815f05-3926-466f-822a-1e20299e5461
This looks like a driver bug. So handle the corner case explicity
to workaround it.

Reviewed By: mravishankar

Differential Revision: https://reviews.llvm.org/D127747