[8u] RFR 8248901: Signed immediate support in .../share/assembler.hpp is broken.

[Zhengyu Gu]

I would like to backport this patch to openjdk8u for parity with Oracle 
8u291.

The original patch does not apply cleanly.

- openjdk8u does not have arm (32-bit) port, striped arm specific 
changes from the patch.
- "precond" macro is not defined in openjdk8u, replaced with assert(...).

Original bug: https://bugs.openjdk.java.net/browse/JDK-8248901
Original patch: https://hg.openjdk.java.net/jdk/jdk/rev/ce8fb40c9174

8u webrev: http://cr.openjdk.java.net/~zgu/JDK-8248901-8u/webrev.00/

Thanks,

-Zhengyu