Add test on bytecode text

Test data should be a Kotlin source file with zero or more comments e.g. of
the form: '// 1 INVOKEVIRTUAL'. The test then checks that the generated
bytecode for this file contains exactly one occurrence of the string
'INVOKEVIRTUAL'

compiler/testData/codegen/bytecodeText/intConstantNullableSafeCall.kt

@@ -0,0 +1,5 @@
+val a : Int? = 10
+
+fun foo() = a?.toString()
+
+// 1 IFNULL