[guards] GVN/NewGVN should propagate guard conditions

[preames]

Bugzilla Link	31523
Version	trunk
OS	Linux
Blocks	#30343
CC	@sanjoy

Extended Description

GVN currently does not have any handling for the llvm.experimental.guard intrinsics. It does support assumes. We should extend the handling to propagate equalities from guards as well.

As a trivial example:
guard (a != null);
...
guard (a != null); // can be removed

Note that we also need to add the same handling to the new memory SSA based NewGVN.

[nikic]

llvm.experimental.guard is deprecated, so I don't think we'll want to support it in NewGVN.