[AVR] Fix a bunch of incorrect assertion messages

These should've been checking whether the immediate is a 6-bit unsigned
integer.

If the immediate was '63', this would cause an assertion error which
shouldn't have occurred.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@289315 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed