[InstCombine] Add explicit data layout to test

To avoid the dependence on the SystemZ target. What is relevant here
is a data layout with preferred alignment for i8.

Author: nikic

llvm/test/Transforms/InstCombine/SystemZ/printf-opt-alignment.ll

@@ -1,8 +1,10 @@
-; RUN: opt < %s --passes=instcombine -S -mtriple=systemz-unknown | FileCheck %s
+; RUN: opt < %s --passes=instcombine -S | FileCheck %s
 ;
 ; Check that string replacements inserted by the instcombiner are properly aligned.
 ; The specific case checked replaces `printf("foo\n")` with `puts("foo")`
 
+target datalayout = "i8:8:16"
+
 @msg1 = constant [17 x i8] c"Alignment Check\0A\00", align 2
 ; CHECK: c"Alignment Check\00", align 2