ARM FastISel fix sext/zext fold

Sign- and zero-extension folding was slightly incorrect because it wasn't checking that the shift on extensions was zero. Further, I recently added AND rd, rn, #255 as a form of 8-bit zero extension, and failed to add the folding code for it.

This patch fixes both issues.

This patch fixes both, and the test should remain the same:
  test/CodeGen/ARM/fast-isel-fold.ll