x86: Fix redundant compare elimination

[BradleyWood]

Redundant compare elimination is a code
generation optimization that attempts to drop
redundant compare instructions before a jump.
This can happen when the the previous
instruction sets condition code flags and is
compared with 0. However, some instructions
such as DIV and IDIV can modify these flags
in undefined ways. This commit lets the code
the code generator know that these instructions
may modify condition code flags. In addition,
the code generator should check if the
instruction that modifies these flags belongs
to the expected node.

Fixes: eclipse-openj9/openj9#17496

[BradleyWood]

jenkins build all

[BradleyWood]

@0xdaryl Can you review?

[0xdaryl]

Can you explain why you restrict the search to only instructions generated from the current node?

Minor nits:

• first sentence in the commit message needs some work to improve grammar
• "code-generation" or "code-generator" should be "code generation" or "code generator" throughout for consistency with rest of code base.

[BradleyWood]

Can you explain why you restrict the search to only instructions generated from the current node?

Minor nits:

• first sentence in the commit message needs some work to improve grammar
• "code-generation" or "code-generator" should be "code generation" or "code generator" throughout for consistency with rest of code base.

node in this case is the node (such as sub or add) that we expect to have set the condition code flags. If any other instructions set those flags and do not belong to that node, removing the comparison would not make sense.

[BradleyWood]

Changed commit message

[0xdaryl]

Jenkins build xlinux,osx,win