Don't use %g0 to materialize 0 directly.

The wired physreg doesn't work on tied operands like on MOVXCC.

Add a README note to fix this later.

llvm-svn: 182225
diff --git a/llvm/test/CodeGen/SPARC/64bit.ll b/llvm/test/CodeGen/SPARC/64bit.ll
index 37dcea9..dd6cee3 100644
--- a/llvm/test/CodeGen/SPARC/64bit.ll
+++ b/llvm/test/CodeGen/SPARC/64bit.ll
@@ -26,7 +26,7 @@
 ;     restore %g0, %g0, %o0
 ;
 ; CHECK: ret_imm0
-; CHECK: or %g0, %g0, %i0
+; CHECK: or %g0, 0, %i0
 define i64 @ret_imm0() {
   ret i64 0
 }