JVM_IR: default-initialize variables visible in the LVT

See KT-36812. Aside from the problem stated there, D8 will throw out the
entire LVT if it sees a variable that has not been written to (and will
generate incorrect SSA if the slot is reused with a different type).

Note: this only fixes a FIR test because it's missing an `else -> throw`
branch, and default initialization satisfies the verifier and masks the
incorrect control flow.

compiler/testData/codegen/bytecodeText/localInitializationLVT/whenStatementVar.kt

@@ -1,6 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
-// TODO KT-36812 Generate proper lifetime intervals for local variables in JVM_IR
-
 fun test(i: Int): Char {
     var c: Char
     when (i) {
@@ -20,4 +17,4 @@ fun test(i: Int): Char {
 }
 
 // 12 ISTORE 1
-// 1 LOCALVARIABLE c C L1 L16 1
+// 1 LOCALVARIABLE c C