RFR: 8202976: Add C1 lea patching support for x86

[Per Liden]

This patch adds C1 support for patching lea instructions on x86. This is 
used in ZGC load barrier stubs, when a load barrier is applied to a 
field load with a not-yet-unknown offset.

Bug: https://bugs.openjdk.java.net/browse/JDK-8202976
Webrev: http://cr.openjdk.java.net/~pliden/8202976/webrev.0

Testing: passes hs-tier{1,2}, this patch has also been used and tested 
in the ZGC repository for more than 3 years.

/Per