string methods intrinsics, place for checks

[Vladimir Kozlov]

I am modifying our string intrinsics to pass char[] pointers and
counters instead of string oops to allow EA eliminate non-escaping
string object (6827605). This will also allow to do some checks
in ideal graph instead of in a mach node encoding. But it may affect
ideal optimizations since it may complicate control flow.

So I need your opinion where I should draw the line.

For example, for String.equals() I compare counters in ideal but
check it for 0 in mach (mach node needs only one counter) since
== 0 is rare case.

Thanks,
Vladimir