Handle If in BCodeBodyBuilder.genCond().

This is more for completeness of `genCond()`. It can now push down
the jumps inside all the control structures for which it makes
sense to do so. It typically applies when the condition of an `if`
or `while` is an `if..else` expression.

It is however not always a win. If the result of the nested `if`s
must be converted to a boolean anyway, doing this will result in
duplication of the conversion to boolean.

A jardiff on the bootstrapped `scala3-compiler.jar` shows 191
additions and 287 deletions, suggesting that, on average, this is
good to have.

Author: sjrd

compiler/src/dotty/tools/backend/jvm/BCodeBodyBuilder.scala

@@ -1570,6 +1570,15 @@ trait BCodeBodyBuilder extends BCodeSkelBuilder {
           emitLocalVarScopes()
           varsInScope = savedScope
 
+        case If(condp, thenp, elsep) =>
+          val innerSuccess = new asm.Label
+          val innerFailure = new asm.Label
+          genCond(condp, innerSuccess, innerFailure, targetIfNoJump = innerSuccess)
+          markProgramPoint(innerSuccess)
+          genCond(thenp, success, failure, targetIfNoJump = innerFailure)
+          markProgramPoint(innerFailure)
+          genCond(elsep, success, failure, targetIfNoJump)
+
         case _ => loadAndTestBoolean()
       }