RFR(S): 8206075: add assertion for unbound assembler Labels for x86

[Liu Xin]

Hi, Community, 
 
Could you please review this small patch?
 
Bug:  https://bugs.openjdk.java.net/browse/JDK-8206075 <https://bugs.openjdk.java.net/browse/JDK-8206075>
CR:  http://cr.openjdk.java.net/~phh/8206075/webrev.00/ <http://cr.openjdk.java.net/~phh/8206075/webrev.00/>
 
Problem:
X86-32/64 will leave an unbound label if UseOnStackReplacement is OFF.
This patch align up x86 with other architectures(ppc, arm).
Add an assertion to the destructor of Label. It  will be wiped out in release build.
 
Previously, hotspot cannot pass this test with assertion on x86-64. 
make run-test TEST=test/hotspot/jtreg/compiler/c1/Test7090976.java
 
If this CR is approved, Paul Hohensee will push it.
 
Thanks,
--lx