RFR: 8213795: Force explicit null check on patching placeholder offset

[Roman Kennke]

With current logic after JDK-8213199, we allow implicit null-checks in
the offset range [-cell_header_size;vm_page_size). When using
Shenandoah, cell_header_size is -8, so we allow [-8;vm_page_size).
Unfortunately, this disables explicit null-checks on -1 which is used as
placeholder for offsets to be patched in C1. This results in weird
asserts later and may actually crash if offset is outside of legal range
for implicit null-checks. We need to force explicit null checks on -1.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8213795
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8213795/webrev.00/

Testing: tier1 and Shenandoah testing

Can I please get a review?

Roman

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