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/coroutines/varValueConflictsWithTableSameSort.kt

@@ -1,6 +1,3 @@
-// This test checks, that different variables occupy the same slot
-// In JVM_IR, however, loop variable's lifetime goes beyond the loop itself, thus the test has no sense in JVM_IR
-// IGNORE_BACKEND: JVM_IR
 // WITH_COROUTINES
 
 import helpers.*