[AVR] XFAIL a set of failing CodeGen tests

There are about 3 underlying bugs causing the tests to fail.

On top of that, some tests just we're 'generic' enough. i.e. 32-bit
registers.

llvm-svn: 294434
diff --git a/llvm/test/CodeGen/Generic/vector.ll b/llvm/test/CodeGen/Generic/vector.ll
index 2d4dc50..9c0cacd 100644
--- a/llvm/test/CodeGen/Generic/vector.ll
+++ b/llvm/test/CodeGen/Generic/vector.ll
@@ -1,6 +1,9 @@
 ; Test that vectors are scalarized/lowered correctly.
 ; RUN: llc < %s
 
+; Bug: PR31898
+; XFAIL: avr
+
 %d8 = type <8 x double>
 %f1 = type <1 x float>
 %f2 = type <2 x float>