cmd/compile: teach prove about bitwise OR operation
For now, only apply the rule if either of arguments are constants. That
would catch a lot of real user code, without slowing down the compiler
with code generated for string comparison (experience in CL 410336).
Updates #57959
Fixes #45928
Change-Id: Ie2e830d6d0d71cda3947818b22c2775bd94f7971
Reviewed-on: https://go-review.googlesource.com/c/go/+/483359
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