JVM_IR don't use Intrinsics.stringPlus for 2-argument concatenation

[dnpetrov]

Using Intrinsics.stringPlus for string concatenation causes performance regressions in HotSpot.

1. It requires boxing for primitive types.
2. It affects string concatenation optimizations in HotSpot.

In fact, it's questionable whether we should use Intrinsics.stringPlus at all.