JVM_IR: handle Nothing and Unit more consistently.

* In blocks, discard the result of any statement that has a return
   type other than void. This was previously done by wrapping each
   statement into an "implicit Unit conversion" that was actually
   compiled down to a stack pop instead. If an expression happened to
   already have type Unit, however, such a conversion was not inserted,
   resulting in a stray reference on the stack. These conversions are
   now redundant and should probably be removed.

 * In assignments and non-exhaustive conditionals, materialize a Unit
   on the stack to avoid depth mismatches that trip up the bytecode
   validator. Because such expressions are generally used at block level
   (and, indeed, the frontend will reject a non-exhaustive conditional
   used as an expression), combined with the above change this results
   in no additional GETSTATIC opcodes, as they are immediately removed
   by the peephole optimizer.

compiler/testData/codegen/bytecodeText/constantConditions/inlineIfFalse.kt

@@ -18,3 +18,6 @@ inline fun inlineCall(predicate: (String?) -> Boolean): Boolean {
 // 0 LINENUMBER 7
 // 0 LINENUMBER 8
 // 1 LINENUMBER 9
+
+// Not actually inlined, so there is a LINENUMBER 7 because the if's body is not considered dead.
+// IGNORE_BACKEND: JVM_IR