Fir2Ir: Set initializers for temporary variables

compiler/testData/ir/irText/expressions/whenCoercedToUnit.fir.txt

@@ -4,6 +4,7 @@ FILE fqName:<root> fileName:/whenCoercedToUnit.kt
     BLOCK_BODY
       BLOCK type=kotlin.Unit origin=WHEN
         VAR IR_TEMPORARY_VARIABLE name:tmp0_subject type:kotlin.Int [val]
+          GET_VAR 'x: kotlin.Int declared in <root>.foo' type=kotlin.Int origin=null
         WHEN type=kotlin.Unit origin=WHEN
           BRANCH
             if: CALL 'public final fun EQEQ (arg0: kotlin.Any?, arg1: kotlin.Any?): kotlin.Boolean declared in kotlin.internal.ir' type=kotlin.Boolean origin=EQEQ